Broaden unsupported-image-input recovery beyond OpenRouter 404s (OpenAI/Anthropic reject with 400)

Follow-up from https://github.com/block/buzz/pull/4896#discussion_r3721468349.

#4896 recovers a turn when a provider rejects image input, but the classifier (`is_unsupported_image_input_error` in `crates/buzz-agent/src/llm.rs`) only matches the OpenRouter/DeepSeek rejection (`No endpoints found that support image input`) and only runs on the 404 arms of `post()` / `openrouter_post()`.

Probed against the real post path: OpenAI- and Claude-style image rejections arrive as **400s**, which are classified as `AgentError::Llm` before the matcher ever runs. So broadening the phrase list alone changes nothing for those providers; real coverage requires:

1. Hoisting the unsupported-image check above the status dispatch (with ordering tests so 401/403/429/5xx handling is unaffected).
2. Per-provider phrases matched against **captured** rejection bodies — not guessed wording. We currently have a verbatim body only for the OpenRouter/DeepSeek case (from the trial run that motivated #4896).

Blocked on: capturing a real image-rejection body from OpenAI and Anthropic (send one image request to a text-only model on each and record status + body verbatim).

Risk note for whoever picks this up: a false positive in this classifier strips images from history on a turn where images were fine — prefer narrow matchers over broad ones like `contains("not support image")`.