feat: add extensible commit filters

[schlotter]

Description

Add convention-agnostic commit filtering hooks to BaseCommitizen.

Custom rules can now override one shared filter_commits method or the
operation-specific filter_commits_before_bump and
filter_commits_before_changelog methods. The hooks run before bump calculation
and changelog generation without changing the existing line-by-line
bump_pattern behavior.

cz version --next USE_GIT_COMMITS uses the bump hook as well, so all
commit-derived version calculations remain consistent. cz check is
intentionally unchanged.

This is an alternative implementation for
commitizen-tools/commitizen#2075
alongside
commitizen-tools/commitizen#2078.
Instead of adding a built-in filtering regex, it provides the generic extension
point proposed in the PR discussion.

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes (GitHub Copilot CLI)

Generated-by: GitHub Copilot CLI following the guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run uv run poe all locally to ensure this change passes linter check and tests
  • Formatting, Ruff, mypy, and commit-message validation passed.
  • The suite passed 1,313 tests, but two pre-existing
    test_bump_pre_commit_changelog cases could not install prettier/node in
    their isolated hook environments because of local TLS certificate errors.
  • A coverage run excluding that network-dependent test function passed all
    1,309 selected tests with 98% coverage.
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

Documentation Changes

• Run uv run poe doc:build locally to ensure the documentation renders correctly
• Check and fix any broken links (internal or external)

The changed documentation also passes the blacken-docs and codespell
pre-commit hooks.

Expected Behavior

Existing rules retain all commits by default, so current bump and changelog
behavior is unchanged.

A Python commit rule can override filter_commits to select commits using the
full commit message. Both bump calculation and changelog generation then process
only the selected commits. A rule can instead override an operation-specific
method when bump and changelog selection should differ.

Steps to Test This Pull Request

1. Create a custom rule that subclasses ConventionalCommitsCz and overrides
   filter_commits to select commits containing Applications: ['AppA'].
2. Add an AppB feat commit and an AppA fix commit after the current version
   tag.
3. Run cz bump --get-next and confirm the next version is a patch rather than a
   minor release.
4. Run cz version --project --next USE_GIT_COMMITS and confirm it reports the
   same patch version.
5. Run cz changelog --dry-run and confirm only the AppA commit is included.
6. Use the built-in rule without an override and confirm all commits remain
   available to bump and changelog processing.

Additional Context

The documentation includes a complete ConventionalCommitsCz plugin example
that reads a plugin-owned app = "AppA" key from [tool.commitizen] and filters
the full Applications: [...] metadata line.

This change does not add a core configuration setting, alter cz check, modify
CHANGELOG.md, or update project versions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.27%. Comparing base (4967717) to head (0a593cd).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2090   +/-   ##
=======================================
  Coverage   98.26%   98.27%           
=======================================
  Files          61       61           
  Lines        2829     2836    +7     
=======================================
+ Hits         2780     2787    +7     
  Misses         49       49           

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[woile]

LGTM
cc @noirbizarre @Lee-W @bearomorphism