chore(deps): update Python quality toolchain

[avksp]

Summary

• update the locked Python quality toolchain to Ruff 0.16.5, Mypy 2.3.1, and Coverage 7.15.4
• keep pyproject.toml, uv.lock, and policy/python-quality.json version-bound
• group future Python quality updates in Dependabot so exact pins and the shared lockfile move atomically
• resolve two stricter Mypy 2.3.1 diagnostics without changing runtime behavior

Validation

• uv lock --check
• full bounded Python quality run: PASS
• Python quality evidence validation: PASS
• 2039 tests under coverage: PASS
• targeted phase-resource, finalization, quality-validator, and workflow tests: 33 PASS
• Dependabot YAML parse: PASS

[avksp]

Independent review on exact head 586e3b3:

• Grok 4.6 xhigh: PASS; no Medium-or-higher defect remains after restoring the historical coverage baseline version.
• OpenCode GLM 5.3: PASS; no Medium-or-higher defects.
• Detached audit worktree remained unchanged.
• Full GitHub CI matrix is green on the reviewed SHA.