Unify pycodestyle checks into ruff

[invisig0th]

Summary

• Remove pycodestyle as a dependency; consolidate all style checks into ruff
• Add all pycodestyle rules from setup.cfg to ruff's select list in pyproject.toml, with line-length = 120 and synapse/vendor/* excluded
• Simplify CI syntax step and pre-commit hook to only run ruff check
• Fix 17 E226 whitespace violations (auto-fixed), suppress 2 intentional E712 == True comparisons with noqa, and suppress W291 in test data containing intentional unicode whitespace
• Update contributing docs to reference ruff instead of pycodestyle

Test plan

• ruff check passes clean from the repo root
• CI syntax step passes with only ruff installed
• Pre-commit hook catches style violations using ruff
• Verify setup.cfg only contains the [vtx_common:github_release] section

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.73%. Comparing base (138083d) to head (3899e11).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4802      +/-   ##
==========================================
- Coverage   97.73%   97.73%   -0.01%     
==========================================
  Files         298      298              
  Lines       62409    62409              
==========================================
- Hits        60995    60994       -1     
- Misses       1414     1415       +1     

Flag	Coverage Δ
linux	97.66% <100.00%> (-0.01%)	⬇️
linux_replay	93.40% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.