Avoid inferring invalid expr types for string annotation (#14447)

## Summary

fixes: #14440

## Test Plan

Add a test case with all the invalid expressions in a string annotation
context.

crates/red_knot_python_semantic/resources/mdtest/annotations/string.md

@@ -189,3 +189,31 @@ reveal_type(d)  # revealed: Foo
 ## Parameter
 
 TODO: Add tests once parameter inference is supported
+
+## Invalid expressions
+
+The expressions in these string annotations aren't valid expressions in this context but we
+shouldn't panic.
+
+```py
+a: "1 or 2"
+b: "(x := 1)"
+c: "1 + 2"
+d: "lambda x: x"
+e: "x if True else y"
+f: "{'a': 1, 'b': 2}"
+g: "{1, 2}"
+h: "[i for i in range(5)]"
+i: "{i for i in range(5)}"
+j: "{i: i for i in range(5)}"
+k: "(i for i in range(5))"
+l: "await 1"
+# error: [forward-annotation-syntax-error]
+m: "yield 1"
+# error: [forward-annotation-syntax-error]
+n: "yield from 1"
+o: "1 < 2"
+p: "call()"
+r: "[1, 2]"
+s: "(1, 2)"
+```

crates/red_knot_python_semantic/src/types/infer.rs

@@ -4374,6 +4374,10 @@ impl<'db> TypeInferenceBuilder<'db> {
                 todo_type!()
             }
 
+            // Avoid inferring the types of invalid type expressions that have been parsed from a
+            // string annotation, as they are not present in the semantic index.
+            _ if self.deferred_state.in_string_annotation() => Type::Unknown,
+
             // Forms which are invalid in the context of annotation expressions: we infer their
             // nested expressions as normal expressions, but the type of the top-level expression is
             // always `Type::Unknown` in these cases.
@@ -4457,7 +4461,6 @@ impl<'db> TypeInferenceBuilder<'db> {
                 self.infer_slice_expression(slice);
                 Type::Unknown
             }
-
             ast::Expr::IpyEscapeCommand(_) => todo!("Implement Ipy escape command support"),
         }
     }