fix: stop prompting basic analysts to call tools they lack

[ethancjackson]

Summary

• Basic WTI/BoC analyst configs attach no tools, but shared system prompts told the model to call search_web (and hard-required set_model_response), which can produce silent empty turns in adk web when the model attempts unavailable tools.
• Split search guidance into supplements appended only by news-capable factories (same pattern as the existing code-exec / forecast-tool supplements).
• Softened structured-output wording to “if set_model_response is available… otherwise return plain JSON,” matching the multitask prompt.

Test plan

• Confirm build_wti_basic_config().instruction / build_boc_basic_config().instruction contain no search_web
• Confirm news configs still include search guidance + recommended queries (WTI) / verification-failure handling (BoC)
• uv run adk web implementations/energy_oil_forecasting/analyst_agent — ask a search-like question on basic; expect a text reply (no silent blank turn from a missing-tool call)
• Spot-check a news-config predictor path still instructs search_web before forecasting

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