fix: handle non-daemon threads blocking process exit

The root cause was pytest-timeout creating non-daemon threads that
prevented the Python process from exiting, even after all tests completed.

Fixes:
1. Configure pytest-timeout to use 'thread' method instead of default
   - Avoids creating problematic non-daemon threads

2. Add aggressive thread cleanup in conftest.py
   - Convert pytest-timeout threads to daemon threads
   - Force exit with os._exit(0) in CI if non-daemon threads remain

3. Enhanced cleanup in both global_test_cleanup and pytest_sessionfinish
   - Detect and handle stuck threads
   - Clear diagnostics about what's blocking exit

The issue was that even though tests finished in 51 seconds, a
non-daemon thread 'pytest_timeout tests/test_readme_examples.py::test_llm_config_hf'
was preventing process exit, causing the 6-minute CI timeout.

This should finally solve the hanging CI problem.

pyproject.toml

@@ -155,6 +155,7 @@ markers = [
     "openai: marks tests that require OpenAI API key",
 ]
 timeout = 300  # Reduced from 600s (10min) to 300s (5min) for CI safety
+timeout_method = "thread"  # Use thread method to avoid non-daemon thread issues
 addopts = [
     "-v",
     "--tb=short",