Some providers (Xiaomi MiMo, some Alibaba endpoints, a long tail of
OpenAI-compatible servers) follow the OpenAI spec strictly and require
tool message `content` to be a string — they reject our list-type
content (text + image_url parts) with HTTP 400 'text is not set' /
'tool message content must be a string'.
Instead of an allowlist of known-good providers (maintenance burden,
guaranteed to miss aggregators like OpenRouter where the underlying
model determines support, not the aggregator name), this lands a
reactive recovery:
1. New `FailoverReason.multimodal_tool_content_unsupported` with a
small pattern list covering the common 400 wordings.
2. `AIAgent._try_strip_image_parts_from_tool_messages` walks the API
message list, downgrades any `role:tool` message whose content is
list-with-image to a plain text summary (preserves text parts) in
place, AND records the active (provider, model) in a session-scoped
`_no_list_tool_content_models` set.
3. `_tool_result_content_for_active_model` short-circuits to a text
summary when (provider, model) is in the cache — so after the first
400 + retry, subsequent screenshots in the same session skip the
round trip entirely.
4. Retry hook in `agent.conversation_loop` mirrors the existing
`image_too_large` recovery: detect the reason, run the helper,
retry once, fall through to the normal error path if no list-type
tool content was actually present.
Cache is transient (per-session) by design — next session retries in
case the provider added support, no persistent state to maintain.
Fixes#27344. Closes#27351 (allowlist approach superseded by reactive
recovery).
