Clean up dead code in agent backends, validate Gemini responses
- Add empty response check in google.py - raises error if model returns
null/empty text instead of emitting finish with result: null
- Move setup_logging() to agents.py instead of importing from backend
- Remove unused functions: setup_logging (all backends), run_prompt (all),
call_filtered_tool (google)
- Fix stale docstrings referencing non-existent AgentSession class
- Remove unused sys import from openai.py
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>