Respect overwrite_existing_annotations for attributes and globals

[Sanjays2402]

Closes #1159

Summary

ApplyTypeAnnotationsVisitor only overwrote existing annotations on function parameters and returns. An attribute or global that already carries an annotation is an AnnAssign, which the transformer never visited, so overwrite_existing_annotations=True had no effect there — patching class X: x: str with a stub class X: x: int left x: str unchanged.

Adds leave_AnnAssign, which (only when overwriting is requested) looks up the qualified name in the stub's attribute annotations and swaps in the stub annotation, recording it in the annotation counts.

Test Plan

New test_annotate_attributes_with_existing_annotations in libcst/codemod/visitors/tests/test_apply_type_annotations.py covers a class attribute, an attribute with a value, and a module-level global. All three fail without the source change and pass with it; libcst/codemod/ is otherwise unchanged (389 passed, with one pre-existing test_codemod_formatter_error_input failure present on a clean checkout too).

This change was prepared with AI assistance; the regression test was run locally and fails without the fix.

[Sanjays2402]

Ping — this one's waiting on workflow approval before CI can run. Would appreciate a maintainer enabling the run whenever there's a spare moment. Open to feedback on the approach.