Xiaomi MiMo (and potentially other providers) support multimodal user
messages but reject list-type tool message content with 400 'text is not
set'. Previously this was handled reactively — the API call would fail,
images would be stripped, and the request retried, losing visual info.
Fix: add supports_vision_tool_messages field to ProviderProfile (default
True). Xiaomi sets it to False. _tool_result_content_for_active_model
now checks this field proactively and returns a text summary instead of
list content, avoiding the round-trip failure entirely.
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