Add a --check mode to the extract command

[ChrisJr404]

Closes #900.

The update command already has a --check mode that verifies catalogs are up to date without writing them, which is very handy in CI. This adds the equivalent for extract.

With --check, messages are extracted from the source as usual, but the output file is not written. Instead the freshly extracted catalog is compared against the existing --output-file, and a non-zero exit code (a BaseError) is raised if:

• the output file does not exist, or
• the extracted template would differ from what is on disk.

If nothing would change, the command logs that the file is up to date and returns normally.

POT-Creation-Date

A fresh extraction always regenerates the POT-Creation-Date header, so a naive comparison would always report a difference. The check therefore ignores that header alone (it copies the existing file's creation date onto the freshly extracted catalog before comparing), mirroring how update --check neutralises the volatile revision date. Any real change to the messages or the other headers is still reported.

The comparison round-trips the extracted catalog through write_po/read_po with the same options that would be used to write it, so the check reflects exactly what would end up on disk.

Usage

$ pybabel extract --check -o messages.pot mypackage/

Tests

Added test_extract_check and test_extract_check_ignores_pot_creation_date in tests/messages/frontend/test_cli.py, covering the up-to-date, changed, and missing-file cases, that the check never writes/creates the output file, and that a differing POT-Creation-Date alone does not trigger a failure. The docs for the extract command are updated as well.

pytest tests/messages passes (389 tests) and ruff check is clean.