Second call site (non-streaming / gateway path) now flattens list-type
content with flatten_message_text before the inline <think> regex and the
surrogate sanitizer, matching the interim-text fix from the prior commit.
Adds regression tests (tests/run_agent/test_66267_multimodal_interim.py)
covering:
- build_assistant_message with list content does not raise TypeError
- inline <think> inside list content is extracted + stripped correctly
- _interim_assistant_visible_text is safe for tool messages (list content)
- duplicate_previous_interim dedup guards against tool messages
Verified the tests fail without the fix (TypeError: expected string... got
'list') and pass with it.