FU-P13-T8: Prevent Web UI port collision from destabilizing MCP sessions

[SoundBlaster]

Description

Fixes an unhandled SystemExit exception from uvicorn's daemon thread when the Web UI port becomes occupied after is_port_available() returns True (TOCTOU race). Previously, run_server() caught OSError but not SystemExit(1), which uvicorn raises internally when port binding fails — this leaked as a PytestUnhandledThreadExceptionWarning in tests and as stderr noise in production.

Changes:

• src/mcpbridge_wrapper/webui/server.py: extended run_server() to also catch SystemExit, printing a clear warning to stderr and returning cleanly so the daemon thread exits without an unhandled exception
• tests/unit/test_main_webui.py: added test_toctou_systemexit_from_uvicorn_does_not_crash_thread to TestPortCollisionHandling — simulates uvicorn raising SystemExit(1) and asserts no exception propagates

Type of Change

• Bug fix

Quality Gates

• 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)

Results:

pytest: 472 passed, 5 skipped — PytestUnhandledThreadExceptionWarning resolved
ruff check src/: All checks passed
coverage: 95.6% (≥90% required)

Documentation Sync

• Documentation changes are synced with DocC catalog (or N/A) — no docs/ files changed

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