Fix extract_messages dry-run behavior

[nightcityblade]

Fixes #910

The setuptools extract_messages command now honors the distutils dry-run flag and returns before creating or overwriting its output catalog. A regression test verifies that an existing POT file remains unchanged.

Tests:

• pytest -q tests/messages (353 passed)
• uvx ruff==0.14.10 check babel/messages/setuptools_frontend.py tests/messages/test_setuptools_frontend.py

[akx]

Erm. Doesn't this mean that passing --dry-run is equivalent to not calling the command at all? That's not the point.

[nightcityblade]

Thanks — you're right. I reworked the implementation so dry-run now performs mapping, extraction, and serialization into an in-memory buffer while leaving the output catalog untouched. The regression test also asserts that extraction actually occurred. The full messages test suite passes (353 tests), and Ruff passes on all changed files.