Polish GTN agent to match agent fleet patterns

Apply the patterns the rest of the agent fleet has converged on:

- Add _validate_query at the start of process(), like every other agent
- Use extract_structured_output() helper instead of the manual hasattr
  result.output / result.data dance
- Pass message_history through _run_with_retry so multi-turn conversations
  benefit from the new structured history channel in BaseGalaxyAgent
- Drop the dead _prepare_prompt override -- the GTN process() never called
  it, and base now strips conversation_history generically anyway
- Collapse the duplicate 'if self._supports_structured_output()' branches
  in _create_agent into one early return
- Strip the noisy what-not-why comments and obvious docstrings throughout
  per project style

No behavior change beyond the multi-turn message_history path, which was
unreachable before.

Author: dannon