fix(benchmarks): guard against empty choices and message=None in LLM eval calls

[qizwiz]

What

Add guards at three LLM evaluation call sites in the EvoAgentBench domain evaluators before accessing choices[0].message.content.

Why

client.chat.completions.create() can return two empty-response shapes:

1. choices = [] — on content-policy rejections, rate-limit errors, or provider failures
2. choices[0].message = None — e.g. Gemini 2.5 Flash (via OpenAI-compatible endpoint) returns HTTP 200 with finish_reason: PROHIBITED_CONTENT and message=None

Both crash with IndexError or AttributeError. The existing try/except blocks catch these as generic "LLM evaluation failed: list index out of range" errors, making benchmark runs hard to diagnose.

Files changed

File	Fix
benchmarks/EvoAgentBench/src/domains/information_retrieval/judge.py	Guard before resp.choices[0].message.content or ""
benchmarks/EvoAgentBench/src/domains/knowledge_work/evaluate.py	Guard before resp.choices[0].message.content
benchmarks/EvoAgentBench/src/domains/reasoning/evaluate.py	Guard before response.choices[0].message.content

# Before
eval_text = resp.choices[0].message.content

# After
if not resp.choices or resp.choices[0].message is None:
    raise ValueError("LLM returned empty or filtered response")
eval_text = resp.choices[0].message.content

Corpus context

Detected by pact (llm_response_unguarded mode), a Z3-verified static analyzer for LLM crash vectors. This pattern was found across 13.8k violations in 800+ repos.