fix: reject non-dict items in the messages array with a clean 400

[shrdgn]

What & why

chat_completions in openfusion/server.py validated that messages is a non-empty list, but never checked that each item in it is an object. That check exists inconsistently elsewhere in the codebase: router.py, cache.py, and estimate.py all guard message access with isinstance(message, dict), but panel.py, pipeline.py, ranked.py, and synthesize.py call message.get(...) directly on every item with no such guard.

A request like:

{"model": "openfusion", "messages": [{"role": "user", "content": "hi"}, "oops"]}

reaches those unguarded call sites and raises an AttributeError: 'str' object has no attribute 'get'. The endpoint's catch-all except Exception turns that into a generic UpstreamError, which defaults to HTTP 502 — the client gets an opaque "upstream" failure for what is really a malformed request that should be a clean 400 invalid_request_error, consistent with the existing messages is required and must be a non-empty array check right above it.

This PR adds the missing isinstance(message, dict) check next to the existing list/non-empty check, so malformed items are rejected at the request boundary before reaching any fusion strategy.

How it was tested

• ruff check . passes
• pytest -q passes (491 passed, no live network)
• New behavior has a test (test_rejects_non_dict_message_item in tests/test_server_pass_through.py)
• Docs updated if config / request surface / defaults changed — N/A, no config/request-surface change, just tightened validation of an already-invalid input
• No secrets, prompts, or response bodies added to logs or metrics
• Quality/cost claims backed by a reproducible bench/run.py number — N/A, not a quality/cost change

Notes for reviewers

Found while auditing the codebase for inconsistent input validation; scoped to just the one missing guard rather than touching panel.py/pipeline.py/ranked.py/synthesize.py, since validating at the API boundary makes the per-module guards unnecessary defense-in-depth rather than required fixes.

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