[ty] Enforce typing.Final (#19178)

## Summary

Emit a diagnostic when a `Final`-qualified symbol is modified. This
first iteration only works for name targets. Tests with TODO comments
were added for attribute assignments as well.

related ticket: https://github.com/astral-sh/ty/issues/158

## Ecosystem impact

Correctly identified [modification of a `Final`
symbol](7b4164a5f2/sphinx/__init__.py (L44))
(behind a `# type: ignore`):
```diff
- warning[unused-ignore-comment] sphinx/__init__.py:44:56: Unused blanket `type: ignore` directive
```
And the same
[here](5471a37e82/src/trio/_core/_run.py (L128)):
```diff
- warning[unused-ignore-comment] src/trio/_core/_run.py:128:45: Unused blanket `type: ignore` directive
```

## Test Plan

New Markdown tests

crates/ty_python_semantic/resources/mdtest/snapshots/final.md_-_`typing.Final`_-_Full_diagnostics_(174fdd8134fb325b).snap

@@ -0,0 +1,42 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: final.md - `typing.Final` - Full diagnostics
+mdtest path: crates/ty_python_semantic/resources/mdtest/type_qualifiers/final.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | from typing import Final
+2 | 
+3 | MY_CONSTANT: Final[int] = 1
+4 | 
+5 | # more code
+6 | 
+7 | MY_CONSTANT = 2  # error: [invalid-assignment]
+```
+
+# Diagnostics
+
+```
+error[invalid-assignment]: Reassignment of `Final` symbol `MY_CONSTANT` is not allowed
+ --> src/mdtest_snippet.py:3:1
+  |
+1 | from typing import Final
+2 |
+3 | MY_CONSTANT: Final[int] = 1
+  | ----------- Original definition
+4 |
+5 | # more code
+6 |
+7 | MY_CONSTANT = 2  # error: [invalid-assignment]
+  | ^^^^^^^^^^^ Reassignment of `Final` symbol
+  |
+info: rule `invalid-assignment` is enabled by default
+
+```

crates/ty_python_semantic/resources/mdtest/type_qualifiers/final.md

@@ -100,9 +100,13 @@ reveal_type(C().FINAL_D)  # revealed: Unknown
 
 ## Not modifiable
 
+### Names
+
 Symbols qualified with `Final` cannot be reassigned, and attempting to do so will result in an
 error:
 
+`mod.py`:
+
 ```py
 from typing import Final, Annotated
 
@@ -114,13 +118,97 @@ FINAL_E: Final[int]
 FINAL_E = 1
 FINAL_F: Final = 1
 
-# TODO: all of these should be errors
-FINAL_A = 2
-FINAL_B = 2
-FINAL_C = 2
-FINAL_D = 2
-FINAL_E = 2
-FINAL_F = 2
+FINAL_A = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_A` is not allowed"
+FINAL_B = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_B` is not allowed"
+FINAL_C = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_C` is not allowed"
+FINAL_D = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_D` is not allowed"
+FINAL_E = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_E` is not allowed"
+FINAL_F = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_F` is not allowed"
+
+def global_use():
+    global FINAL_A, FINAL_B, FINAL_C, FINAL_D, FINAL_E, FINAL_F
+    FINAL_A = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_A` is not allowed"
+    FINAL_B = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_B` is not allowed"
+    FINAL_C = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_C` is not allowed"
+    FINAL_D = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_D` is not allowed"
+    FINAL_E = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_E` is not allowed"
+    FINAL_F = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_F` is not allowed"
+
+def local_use():
+    # These are not errors, because they refer to local variables
+    FINAL_A = 2
+    FINAL_B = 2
+    FINAL_C = 2
+    FINAL_D = 2
+    FINAL_E = 2
+    FINAL_F = 2
+
+def nonlocal_use():
+    X: Final[int] = 1
+    def inner():
+        nonlocal X
+        # TODO: this should be an error
+        X = 2
+```
+
+`main.py`:
+
+```py
+from mod import FINAL_A, FINAL_B, FINAL_C, FINAL_D, FINAL_E, FINAL_F
+
+FINAL_A = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_A` is not allowed"
+FINAL_B = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_B` is not allowed"
+FINAL_C = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_C` is not allowed"
+FINAL_D = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_D` is not allowed"
+FINAL_E = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_E` is not allowed"
+FINAL_F = 2  # error: [invalid-assignment] "Reassignment of `Final` symbol `FINAL_F` is not allowed"
+```
+
+### Attributes
+
+Assignments to attributes qualified with `Final` are also not allowed:
+
+```py
+from typing import Final
+
+class C:
+    FINAL_A: Final[int] = 1
+    FINAL_B: Final = 1
+
+    def __init__(self):
+        self.FINAL_C: Final[int] = 1
+        self.FINAL_D: Final = 1
+
+# TODO: these should be errors (that mention `Final`)
+C.FINAL_A = 2
+# error: [invalid-assignment] "Object of type `Literal[2]` is not assignable to attribute `FINAL_B` of type `Literal[1]`"
+C.FINAL_B = 2
+
+# TODO: these should be errors (that mention `Final`)
+c = C()
+c.FINAL_A = 2
+# error: [invalid-assignment] "Object of type `Literal[2]` is not assignable to attribute `FINAL_B` of type `Literal[1]`"
+c.FINAL_B = 2
+c.FINAL_C = 2
+c.FINAL_D = 2
+```
+
+## Mutability
+
+Objects qualified with `Final` *can be modified*. `Final` represents a constant reference to an
+object, but that object itself may still be mutable:
+
+```py
+from typing import Final
+
+class C:
+    x: int = 1
+
+FINAL_C_INSTANCE: Final[C] = C()
+FINAL_C_INSTANCE.x = 2
+
+FINAL_LIST: Final[list[int]] = [1, 2, 3]
+FINAL_LIST[0] = 4
 ```
 
 ## Too many arguments
@@ -168,4 +256,18 @@ class C:
     NO_RHS: Final
 ```
 
+## Full diagnostics
+
+<!-- snapshot-diagnostics -->
+
+```py
+from typing import Final
+
+MY_CONSTANT: Final[int] = 1
+
+# more code
+
+MY_CONSTANT = 2  # error: [invalid-assignment]
+```
+
 [`typing.final`]: https://docs.python.org/3/library/typing.html#typing.Final