feat: add PEP 751 version provider with pylock.toml support

[josix]

Description

Introduce Pep751Provider that extends Pep621Provider to update version strings in pylock*.toml lock files alongside pyproject.toml.

Checklist

• I have read the contributing guidelines

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

• Yes, Claude Code w/ plugin (https://github.com/josix/agent-flow/)

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
• 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 locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external)

Expected Behavior

Steps to Test This Pull Request

Additional Context

Fix #1397

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.00%. Comparing base (33223b4) to head (7616956).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1896      +/-   ##
==========================================
+ Coverage   97.99%   98.00%   +0.01%     
==========================================
  Files          60       61       +1     
  Lines        2689     2711      +22     
==========================================
+ Hits         2635     2657      +22     
  Misses         54       54              

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