Fixes #21030: Undefined names used as decorators on top of TypedDict are not reported

[pranavmanglik]

This PR ensures that decorators applied to a TypedDict class definition are correctly analyzed for undefined names.

Previously, the semantic analyzer's special-case handling for TypedDict (analyze_typeddict_classdef) was returning early without visiting the decorators list in the ClassDef. This caused mypy to silently ignore invalid names, attributes, or subscripted expressions used as decorators on these classes.
Changes

mypy/semanal.py: Updated analyze_typeddict_classdef to iterate through defn.decorators and call accept(self) on each. This triggers the standard semantic analysis and name-resolution logic for those expressions.

test-data/unit/check-typeddict.test: Added a new data-driven test case testTypedDictDecoratorUndefinedNames covering:

    Simple undefined names (@abc)

    Undefined attributes (@efg.hij)

    Undefined names within subscripts (@klm[nop])

Checklist:

[x] Read the [Contributing Guidelines](https://github.com/python/mypy/blob/master/CONTRIBUTING.md)

[x] Add tests for all changed behaviour.

[x] Make sure CI passes.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅