Add request preprocessing and response postprocessing hooks to TriviaAgentLoop

[andreisavu]

Summary

This PR adds extensibility hooks to TriviaAgentLoop that allow subclasses to transform requests before agent processing and responses after agent execution. This enables custom validation, normalization, formatting, and enrichment logic without modifying the core loop implementation.

Key Changes

• Added preprocess_request() method: Allows subclasses to transform incoming TriviaRequest objects before they are processed by the agent. Default implementation returns the request unchanged.
• Added postprocess_response() method: Allows subclasses to transform outgoing TriviaResponse objects after agent execution. Default implementation returns the response unchanged.
• Overridden execute() method: Implements the preprocessing/postprocessing pipeline by:
  • Calling preprocess_request() on the incoming request
  • Delegating to parent MainLoop.execute() with the preprocessed request
  • Calling postprocess_response() on the response output (if present)
  • Safely handling cases where response output is None
• Updated imports: Added Mapping from collections.abc, and Budget, PromptResponse, and Heartbeat from weakincentives
• Enhanced documentation: Updated class docstring to document the new preprocessing/postprocessing capabilities and added notes to the prepare() method explaining the execution flow

Implementation Details

• The preprocessing occurs in execute() before prepare() is called, ensuring consistent request handling throughout the execution flow
• Postprocessing only applies when prompt_response.output is not None, preventing unnecessary processing of error responses
• Both hooks use the standard override pattern, making them easy to extend in subclasses
• Comprehensive test coverage includes default behavior, custom implementations, and edge cases

Testing

Added three new test classes with 8 test cases covering:

• Default behavior (requests/responses returned unchanged)
• Custom subclass implementations
• Integration with the execute() method
• Edge case handling (None output)