fix: guard choices access before indexing in LLM API calls (10 files)

[qizwiz]

Summary

LLM APIs (OpenAI, Groq, and OpenAI-compatible endpoints) can return an empty choices list in several legitimate scenarios:

• Content filtering triggered (safety system returns no completions)
• Rate-limit or quota errors during streaming
• Network-level partial responses
• Certain model configurations with n=0

Accessing response.choices[0] without a length check raises IndexError in these cases, turning a recoverable API error into an unhandled crash.

This PR adds an explicit if not <response>.choices guard before every unguarded choices[0] access across 10 files:

File	Line	Fix
3p-integrations/groq/.../groq-quickstart-conversational-chatbot/main.py	35	continue loop iteration
3p-integrations/groq/.../presidential-speeches-rag-with-pinecone/main.py	65	return ""
3p-integrations/groq/.../text-to-sql-json-mode/main.py	33	return ""
3p-integrations/groq/.../verified-sql-function-calling/main.py	135	return None
end-to-end-use-cases/Contextual-Chunking-RAG/helper.py	30, 40	return ""
end-to-end-use-cases/Multi-Modal-RAG/scripts/final_demo.py	115, 184	return fallback
end-to-end-use-cases/github_triage/llm.py	39	inline ternary guard
end-to-end-use-cases/research_paper_analyzer/research_analyzer.py	98	return []
end-to-end-use-cases/structured_parser/src/utils.py	456	raise ValueError
getting-started/inference/api_inference/api_inference.py	40	return ""

Found by pact static analysis (llm_response_unguarded mode), which uses a Z3 Fixedpoint Datalog engine to track unguarded LLM API response accesses across Python call graphs.

Test plan

• Verify pact reports 0 llm_response_unguarded violations on patched files
• Run existing example tests if any exist in CI

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[qizwiz]

Update: guard strengthened after live reproduction (commit 7e52db5)

While building a proof script, we found the original guard (if not response.choices) is incomplete for one additional failure mode.

Gemini 2.5 Flash (OpenAI-compatible endpoint) returns HTTP 200 with choices[0].message = None when its safety filter blocks a request — finish_reason: 'content_filter: PROHIBITED_CONTENT'. The choices list has one entry so the length guard passes, but accessing .message.content raises AttributeError: 'NoneType' object has no attribute 'content'.

# previous guard — still crashes on Gemini content filtering
if not response.choices:
    return ""
return response.choices[0].message.content  # AttributeError if message is None

# comprehensive fix (this commit)  
if not response.choices or response.choices[0].message is None:
    return ""
return response.choices[0].message.content  # safe

All 10 files updated. This covers: empty choices list (any provider), null message on content filter (Gemini and potentially others).

Note: still waiting on CLA confirmation before this can be reviewed — will sign at code.facebook.com/cla.

[qizwiz]

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