fix(agent): recover from providers rejecting list-type tool content (#27344) (#30259)

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 Closes #27351 (allowlist approach superseded by reactive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).

agent/conversation_loop.py

@@ -989,6 +989,7 @@ def run_conversation(
         copilot_auth_retry_attempted=False
         thinking_sig_retry_attempted = False
         image_shrink_retry_attempted = False
+        multimodal_tool_content_retry_attempted = False
         oauth_1m_beta_retry_attempted = False
         llama_cpp_grammar_retry_attempted = False
         has_retried_429 = False
@@ -2060,6 +2061,31 @@ def run_conversation(
                             "or shrink didn't reduce size; surfacing original error."
                         )
 
+                # Multimodal-tool-content recovery: providers that follow
+                # the OpenAI spec strictly (tool message content must be a
+                # string) reject our list-type content with a 400.  Strip
+                # image parts from any list-type tool messages, mark the
+                # (provider, model) as no-list-tool-content for the rest
+                # of this session so future tool results preemptively
+                # downgrade, and retry once.  See issue #27344.
+                if (
+                    classified.reason == FailoverReason.multimodal_tool_content_unsupported
+                    and not multimodal_tool_content_retry_attempted
+                ):
+                    multimodal_tool_content_retry_attempted = True
+                    if agent._try_strip_image_parts_from_tool_messages(api_messages):
+                        agent._vprint(
+                            f"{agent.log_prefix}📐 Provider rejected list-type tool content — "
+                            f"downgraded screenshots to text and retrying...",
+                            force=True,
+                        )
+                        continue
+                    else:
+                        logger.info(
+                            "multimodal-tool-content recovery: no list-type tool "
+                            "messages with image parts found; surfacing original error."
+                        )
+
                 # Anthropic OAuth subscription rejected the 1M-context beta
                 # header ("long context beta is not yet available for this
                 # subscription"). Disable the beta for the rest of this