Enable coverage reporting for test code

[2253781369]

Summary

Remove test files from the coverage omit list so that test code is included in coverage tracking. This helps detect tests that are always skipped — they would show 0% coverage.

Changes

• pyproject.toml: Remove src/towncrier/test/* from coverage omit list
• noxfile.py: Update coverage_report session to show separate production and test coverage reports
• .github/workflows/ci.yml: CI now reports full coverage (including tests) to the step summary, while still enforcing 100% coverage for production code only
• src/towncrier/newsfragments/711.misc: Required changelog fragment

Fixes #711

[adiroiban]

Is this AI generated without any review from a person?

If this was previously reviewed by a person, please double-check the diff.
it looks like this introduces newline changes.

I am closing this for now since I suspect that this is AI spam, but we can reopen it.

[glyph]

I've blocked this user. In the future I think we need to be aggressive about such bans because otherwise we leave a vector for abuse open; they can request reconsideration outside of Github if they feel it's unfair.