FU-P13-T15: Restore broker same-UID client acceptance when peer credential APIs are unavailable

[SoundBlaster]

Description

Implement FU-P13-T15 by fixing broker peer-credential resolution on platforms where the previous fallback raised Errno 42 (Protocol not available) and caused -32003 UID mismatch for same-user local clients.

What changed:

• Refactored _get_peer_uid() to use platform-aware resolution order:
  • getpeereid()
  • LOCAL_PEERCRED
  • SO_PEERCRED
• Removed hard-coded Linux SO_PEERCRED fallback constant usage on unsupported platforms.
• Added focused unit coverage for credential-resolution paths and fail-closed behavior.
• Archived FU-P13-T15 FLOW artifacts and review report.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactoring
• CI/CD improvement

Quality Gates

Before submitting, ensure all quality gates pass:

make check

Or run individually:

• make test - All tests pass with ≥90% coverage
• make lint - No linting errors
• make format - Code is properly formatted
• make typecheck - Type checking passes
• make doccheck - Documentation is synced with DocC (if docs changed)

Documentation Sync

If you modified files in docs/, ensure corresponding DocC files are also updated:

docs/ file	DocC file
docs/installation.md	mcpbridge-wrapper.docc/Installation.md
docs/cursor-setup.md	mcpbridge-wrapper.docc/CursorSetup.md
docs/claude-setup.md	mcpbridge-wrapper.docc/ClaudeCodeSetup.md
docs/codex-setup.md	mcpbridge-wrapper.docc/CodexCLISetup.md
docs/troubleshooting.md	mcpbridge-wrapper.docc/Troubleshooting.md
docs/architecture.md	mcpbridge-wrapper.docc/Architecture.md
docs/environment-variables.md	mcpbridge-wrapper.docc/EnvironmentVariables.md
README.md	mcpbridge-wrapper.docc/mcpbridge-wrapper.md

• Documentation changes are synced with DocC catalog (or N/A)

Testing

• Added/updated tests for new functionality
• All tests pass locally
• Manually tested the changes

Checklist

• Code follows the project's style guidelines
• Self-review completed
• Comments added for complex code
• Documentation updated (if needed)
• No new warnings generated
• PR title is descriptive