A PNG or http(s) image is HTTP 500 on the Qwen3-VL and dots3-note chat seams, not 400

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A PNG, a JPEG, or an http(s) image URL sent to a Qwen3-VL or dots3-note server comes back as HTTP 500, not 400.

oai::DefaultImageCodec refuses container formats by design — the PNG/JPEG→RGB decode is a named residual, and refusing is the correct behaviour under AGENTS.md §"Shared seams". The defect is the STATUS, not the refusal. A request the server cannot serve because of the request's own content is a client error; a 500 tells the caller to retry and tells an operator to go looking for a crash that did not happen.

Where it comes from:

• src/vllm/entrypoints/openai/mm_chat_qwen3vl.cpp:64 and :76 throw a bare std::runtime_error, which falls through to the generic handler and becomes 500.
• src/vllm/entrypoints/openai/mm_chat_dots3note.cpp is inconsistent with itself: it converts to vllm::v1::InputValidationError at :273, with a comment that says exactly why ("a bare runtime_error falls through"), and still throws std::runtime_error at :379.

src/vllm/entrypoints/openai/mm_chat_deepseek_v4.cpp does it correctly as of #2411 W5: it catches the codec's std::invalid_argument and the data-URI failures and re-throws vllm::v1::InputValidationError, which the server renders as 400 with a message naming the unsupported container format.

Found while wiring the DeepSeek-V4 vision seam (#2411 W5). That wave fixed its own seam and deliberately did not widen scope into the other two, which is why this issue exists rather than a silent drive-by edit.

Suggested shape: the conversion belongs in one place rather than three. Either the install's catch normalises a codec refusal, or the codec throws a type the seams do not each have to remember to translate. Whichever is chosen, the fix wants a case per seam that asserts the status code, because the current dots3-note file shows that knowing the rule is not enough to apply it consistently.

Start with AGENTS.md §"Shared seams", then compare the throw sites in src/vllm/entrypoints/openai/mm_chat_qwen3vl.cpp and mm_chat_dots3note.cpp with the handling in mm_chat_deepseek_v4.cpp. Trace the existing seam tests and add one case for each affected seam; done means PNG, JPEG, and http(s) image refusals return HTTP 400 with an explanatory message rather than 500.