fix(buzz-agent): recover from 400-shaped image rejections; unbound benchmark agent rounds

[tlongwell-block]

Problem

Two failure modes from the tb21-glm52-crusoe-1 benchmark run (GLM-5.2 solo, TB2.1) wedged or killed 13 of 89 trials without the model being at fault:

1. Conversation poisoning on text-only endpoints. Crusoe's serverless crusoeai/GLM-5.2-NVFP4 rejects any request whose history contains an image with 400: ... is not a multimodal model. The recovery machinery for exactly this case already exists — AgentError::UnsupportedImageInput → replace_unsupported_images() strips the image blocks, marks the tool result as an error, substitutes a text placeholder, and continues the turn. But classification only matched OpenRouter's 404 body (no endpoints found that support image input) and was only consulted on the 404 arms. The Crusoe 400 fell through to terminal AgentError::Llm: the image stayed in history, every subsequent call failed identically, buzz-acp rode its 10-retry ladder (~40 min), and the trial idled to budget death. Measured blast radius: 8 trials wedged, 12.7h aggregate idle-after-poison.

2. Bounded agent rounds in benchmark trials. The harness default DEFAULT_MAX_AGENT_ROUNDS = 32 ended solo trials mid-work when turns rotated (thinking-heavy models hit max_tokens rotation fast; 4 trials died this way). Benchmark trials already have a wall-clock budget as the real limit — the round cap only converts recoverable rotation into trial death.

Fix

• is_unsupported_image_input_error() also matches the verbatim is not a multimodal model body. Matcher stays deliberately tight (same doctrine as is_context_length_error): misclassifying a generic 400 as recoverable would mutate history for an error that removing images cannot fix.
• Both status ladders — shared post() and openrouter_post() — consult it on their 400 arms and return the typed UnsupportedImageInput (OpenAI-compatible providers report this as 400; a BYOK/passthrough upstream can surface the provider's own 400 through OpenRouter).
• Harness DEFAULT_MAX_AGENT_ROUNDS → 0 (unbounded — BUZZ_AGENT_MAX_ROUNDS=0 is the agent config's documented unbounded value). Per-agent budget.max_calls in manifests still overrides.

Acceptance

• A 400 with the image-rejection body reaches the existing image-strip recovery path instead of wedging the session — asserted through complete() (covers the return path into the convergence mapper) and at the openrouter_post terminal, both proving single-attempt (a deterministic capability rejection must never be retried).
• Ordinary 400s stay terminal AgentError::Llm (existing negative tests unchanged).
• Benchmark trials run unbounded rounds by default; python tests updated for 0-is-legal with a negative arm at -1.

Verification

• cargo test -p buzz-agent: 427 + 18 + 20 + 15 + 8 + 1 + 48 passed, 0 failed (full package, 3 consecutive clean runs)
• cargo clippy -p buzz-agent --all-targets, cargo fmt --check: clean
• uv run --extra dev pytest tests/ in harbor-buzz-orchestra: 35 passed
• Pre-push hooks (full workspace rust-tests + desktop-tauri-checks) green on rustc 1.95.0 at head b043860