Fix uvx resource packaging issues for OpenRouter functionality

Resolves issues #203, #186, #206, #185 where OpenRouter model registry
completely failed to load in uvx installations due to inaccessible
conf/custom_models.json file.

Changes:
- Implement multiple path resolution strategy in OpenRouterModelRegistry
  - Development: Path(__file__).parent.parent / "conf" / "custom_models.json"
  - UVX working dir: Path("conf/custom_models.json")
  - Current working dir: Path.cwd() / "conf" / "custom_models.json"
- Add importlib-resources fallback for Python < 3.9 compatibility
- Add comprehensive test suite for path resolution scenarios
- Ensure graceful handling when config files are missing

The fix restores full OpenRouter functionality (15 models, 62+ aliases)
for users installing via uvx while maintaining backward compatibility
for development and explicit config scenarios.

Tested: All path resolution scenarios pass, OpenRouter models load correctly

requirements.txt

@@ -3,6 +3,7 @@ google-genai>=1.19.0
 openai>=1.55.2  # Minimum version for httpx 0.28.0 compatibility
 pydantic>=2.0.0
 python-dotenv>=1.0.0
+importlib-resources>=5.0.0; python_version<"3.9"
 
 # Development dependencies (install with pip install -r requirements-dev.txt)
 # pytest>=7.4.0