Add BaseRunner class and run() method to Python SDK
- Add BaseRunner as the new base class with run() method
- BasePredictor is now an alias for BaseRunner (backwards compat)
- get_predict() prefers run() over predict(); errors if both defined
- Inspector (schema generation) detects run() or predict() on classes
- Export BaseRunner from cog package
- Add tests for BaseRunner, run(), and dispatch logic
