[ty] highlight the argument in static_assert error messages (#19426)

Closes https://github.com/astral-sh/ty/issues/209.

Before:
```
error[static-assert-error]: Static assertion error: custom message
 --> test.py:2:1
  |
1 | from ty_extensions import static_assert
2 | static_assert(3 > 4, "custom message")
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
```

After:
```
error[static-assert-error]: Static assertion error: custom message
 --> test.py:2:1
  |
1 | from ty_extensions import static_assert
2 | static_assert(3 > 4, "custom message")
  | ^^^^^^^^^^^^^^-----^^^^^^^^^^^^^^^^^^^
  |               |
  |               Inferred type of argument is `Literal[False]`
  |
```

crates/ty_python_semantic/resources/mdtest/directives/cast.md

@@ -1,5 +1,7 @@
 # `cast`
 
+## Behavior
+
 `cast()` takes two arguments, one type and one value, and returns a value of the given type.
 
 The (inferred) type of the value and the given type do not need to have any correlation.
@@ -78,3 +80,15 @@ def f(x: Any, y: Unknown, z: Any | str | int):
 
     e = cast(str | int | Any, z)  # error: [redundant-cast]
 ```
+
+## Diagnostic snapshots
+
+<!-- snapshot-diagnostics -->
+
+```py
+import secrets
+from typing import cast
+
+# error: [redundant-cast] "Value is already of type `int`"
+cast(int, secrets.randbelow(10))
+```

crates/ty_python_semantic/resources/mdtest/snapshots/cast.md_-_`cast`_-_Diagnostic_snapshots_(91dd3d45b6d7f2c8).snap

@@ -0,0 +1,34 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: cast.md - `cast` - Diagnostic snapshots
+mdtest path: crates/ty_python_semantic/resources/mdtest/directives/cast.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | import secrets
+2 | from typing import cast
+3 | 
+4 | # error: [redundant-cast] "Value is already of type `int`"
+5 | cast(int, secrets.randbelow(10))
+```
+
+# Diagnostics
+
+```
+warning[redundant-cast]: Value is already of type `int`
+ --> src/mdtest_snippet.py:5:1
+  |
+4 | # error: [redundant-cast] "Value is already of type `int`"
+5 | cast(int, secrets.randbelow(10))
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+info: rule `redundant-cast` is enabled by default
+
+```

crates/ty_python_semantic/resources/mdtest/snapshots/type_api.md_-_Type_API_(`ty_extens…_-_Diagnostic_snapshots_(be112fd8b8cfdc28).snap

@@ -0,0 +1,104 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: type_api.md - Type API (`ty_extensions`) - Diagnostic snapshots
+mdtest path: crates/ty_python_semantic/resources/mdtest/type_api.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from ty_extensions import static_assert
+ 2 | import secrets
+ 3 | 
+ 4 | # a passing assert
+ 5 | static_assert(1 < 2)
+ 6 | 
+ 7 | # evaluates to False
+ 8 | # error: [static-assert-error]
+ 9 | static_assert(1 > 2)
+10 | 
+11 | # evaluates to False, with a message as the second argument
+12 | # error: [static-assert-error]
+13 | static_assert(1 > 2, "with a message")
+14 | 
+15 | # evaluates to something falsey
+16 | # error: [static-assert-error]
+17 | static_assert("")
+18 | 
+19 | # evaluates to something ambiguous
+20 | # error: [static-assert-error]
+21 | static_assert(secrets.randbelow(2))
+```
+
+# Diagnostics
+
+```
+error[static-assert-error]: Static assertion error: argument evaluates to `False`
+  --> src/mdtest_snippet.py:9:1
+   |
+ 7 | # evaluates to False
+ 8 | # error: [static-assert-error]
+ 9 | static_assert(1 > 2)
+   | ^^^^^^^^^^^^^^-----^
+   |               |
+   |               Inferred type of argument is `Literal[False]`
+10 |
+11 | # evaluates to False, with a message as the second argument
+   |
+info: rule `static-assert-error` is enabled by default
+
+```
+
+```
+error[static-assert-error]: Static assertion error: with a message
+  --> src/mdtest_snippet.py:13:1
+   |
+11 | # evaluates to False, with a message as the second argument
+12 | # error: [static-assert-error]
+13 | static_assert(1 > 2, "with a message")
+   | ^^^^^^^^^^^^^^-----^^^^^^^^^^^^^^^^^^^
+   |               |
+   |               Inferred type of argument is `Literal[False]`
+14 |
+15 | # evaluates to something falsey
+   |
+info: rule `static-assert-error` is enabled by default
+
+```
+
+```
+error[static-assert-error]: Static assertion error: argument of type `Literal[""]` is statically known to be falsy
+  --> src/mdtest_snippet.py:17:1
+   |
+15 | # evaluates to something falsey
+16 | # error: [static-assert-error]
+17 | static_assert("")
+   | ^^^^^^^^^^^^^^--^
+   |               |
+   |               Inferred type of argument is `Literal[""]`
+18 |
+19 | # evaluates to something ambiguous
+   |
+info: rule `static-assert-error` is enabled by default
+
+```
+
+```
+error[static-assert-error]: Static assertion error: argument of type `int` has an ambiguous static truthiness
+  --> src/mdtest_snippet.py:21:1
+   |
+19 | # evaluates to something ambiguous
+20 | # error: [static-assert-error]
+21 | static_assert(secrets.randbelow(2))
+   | ^^^^^^^^^^^^^^--------------------^
+   |               |
+   |               Inferred type of argument is `int`
+   |
+info: rule `static-assert-error` is enabled by default
+
+```

crates/ty_python_semantic/resources/mdtest/type_api.md

@@ -266,6 +266,34 @@ shouted_message = "A custom message".upper()
 static_assert(False, shouted_message)
 ```
 
+## Diagnostic snapshots
+
+<!-- snapshot-diagnostics -->
+
+```py
+from ty_extensions import static_assert
+import secrets
+
+# a passing assert
+static_assert(1 < 2)
+
+# evaluates to False
+# error: [static-assert-error]
+static_assert(1 > 2)
+
+# evaluates to False, with a message as the second argument
+# error: [static-assert-error]
+static_assert(1 > 2, "with a message")
+
+# evaluates to something falsey
+# error: [static-assert-error]
+static_assert("")
+
+# evaluates to something ambiguous
+# error: [static-assert-error]
+static_assert(secrets.randbelow(2))
+```
+
 ## Type predicates
 
 The `ty_extensions` module also provides predicates to test various properties of types. These are

crates/ty_python_semantic/src/types/function.rs

@@ -1264,28 +1264,35 @@ impl KnownFunction {
                     if truthiness.is_always_true() {
                         return;
                     }
-                    if let Some(message) = message
+                    let mut diagnostic = if let Some(message) = message
                         .and_then(Type::into_string_literal)
                         .map(|s| s.value(db))
                     {
-                        builder.into_diagnostic(format_args!("Static assertion error: {message}"));
+                        builder.into_diagnostic(format_args!("Static assertion error: {message}"))
                     } else if *parameter_ty == Type::BooleanLiteral(false) {
                         builder.into_diagnostic(
                             "Static assertion error: argument evaluates to `False`",
-                        );
+                        )
                     } else if truthiness.is_always_false() {
                         builder.into_diagnostic(format_args!(
                             "Static assertion error: argument of type `{parameter_ty}` \
                             is statically known to be falsy",
                             parameter_ty = parameter_ty.display(db)
-                        ));
+                        ))
                     } else {
                         builder.into_diagnostic(format_args!(
                             "Static assertion error: argument of type `{parameter_ty}` \
                             has an ambiguous static truthiness",
                             parameter_ty = parameter_ty.display(db)
-                        ));
+                        ))
-                    }
+                    };
+                    diagnostic.annotate(
+                        Annotation::secondary(context.span(&call_expression.arguments.args[0]))
+                            .message(format_args!(
+                                "Inferred type of argument is `{}`",
+                                parameter_ty.display(db)
+                            )),
+                    );
                 }
             }