Classify malformed-multimodal rejects as invalid_request

[merrymercy]

Summary

• When a multimodal request fails validation (e.g. bad base64), return a structured error with type=invalid_request_error and code=400 instead of a bare error message.
• Detect client disconnects in the streaming path and exit cleanly instead of emitting a 400.
• In the multimodal base processor, re-raise ValueError separately from RuntimeError so callers can distinguish bad input (4xx) from internal failures (5xx), and chain the original exception via from e.

Original commits

• 1bce350b1

Test plan

• Send a streaming request with a malformed base64 image and verify the SSE error event contains "type": "invalid_request_error" and "code": 400.
• Send a streaming multimodal request and disconnect the client mid-flight; verify the server logs an info-level disconnect message and does not emit a 400 error event.
• Send a valid multimodal request and confirm it still succeeds end-to-end.

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CI States

Latest PR Test (Base): 🚫 Run #27066493925
Latest PR Test (Extra): ❌ Run #27066493871

[merrymercy]

/tag-and-rerun-ci

[gemini-code-assist]

Code Review

This pull request improves error handling in the HTTP server and multimodal processors. Specifically, it handles client disconnects gracefully during streaming, structures streaming ValueError responses as 400 errors, and propagates ValueErrors during multimodal data loading to avoid 500 server errors. Feedback highlights two main areas for improvement: first, truncating the raw data string in base_processor.py exception messages to prevent log bloat and memory overhead from large base64 payloads; second, unifying the error response format between the streaming and non-streaming paths in http_server.py to ensure API consistency.

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