Remove unnecessary custom code

[pirate]

Summary by cubic

Unifies model typing across the types package and simplifies docs, with no behavior changes.

• Refactors

  • Added OptionsModel (Union[ModelConfigParam, str]) in session_extract_params.py and exported via __all__.
  • Switched Options.model to OptionsModel in both session_extract_params.py and session_observe_params.py.
  • Updated AgentConfig.model in session_execute_params.py to use AgentConfigModel with a clearer description.
  • Shortened the model_name docstring in session_start_params.py.

• Dependencies

  • Bumped stagehand to 3.19.4 in uv.lock.

^{Written for commit c46c1ef. Summary will update on new commits. Review in cubic}