fix: skip TBD changelog entries in docs generator

[c-vigo]

Description

Fixes release-branch CI failures where generate-docs rewrote README.md to an unreleased TBD version after prepare-release.
The changelog parser in docs/generate.py now selects the latest dated release entry, and regression tests ensure TBD entries are skipped.

Type of Change

• fix -- Bug fix
• docs -- Documentation only
• chore -- Maintenance task (deps, config, etc.)
• refactor -- Code restructuring (no behavior change)
• test -- Adding or updating tests
• ci -- CI/CD pipeline changes
• build -- Build system or dependency changes
• revert -- Reverts a previous commit
• style -- Code style (formatting, whitespace)

Modifiers

• Breaking change (!) -- This change breaks backward compatibility

Changes Made

• docs/generate.py
  • Added date-aware filtering for changelog headings so version/date extraction ignores TBD entries.
  • Updated get_version_from_changelog() and get_release_date_from_changelog() to pick the first heading containing a concrete YYYY-MM-DD date.
• tests/test_utils.py
  • Added regression tests in both changelog parser test classes to verify ## [x.y.z] - TBD headings are skipped.
  • Added a test helper to point parser functions at a temporary changelog fixture.
• CHANGELOG.md
  • Added a ### Fixed entry in ## [0.3.0] - TBD for issue #271.

Changelog Entry

Fixed

• generate-docs picks up unreleased TBD version on release branches (#271)
  • get_version_from_changelog() and get_release_date_from_changelog() now skip entries without a concrete release date

Testing

• Tests pass locally (just test)
• Manual testing performed (describe below)

Manual Testing Details

• uv run pytest tests/test_utils.py -k "skips_tbd_entry" (RED before fix; two failures reproduced)
• uv run pytest tests/test_utils.py -k "changelog" (GREEN after fix; passing)
• uv run pre-commit run generate-docs --all-files (docs regenerated without remaining diffs)

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly (edit docs/templates/, then run just docs)
• I have updated CHANGELOG.md in the [Unreleased] section (and pasted the entry above)
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Additional Notes

This branch targets release/0.3.0 to unblock the release pipeline affected by PR #270 checks.

Refs: #271