Some OpenAI-compatible providers (NVIDIA NIM + qwen3.5) return a string
for model_extra instead of a dict. The falsy fallback (x or {}) treats a
truthy non-empty string as the value and calls .get() on it, raising
AttributeError and turning every tool call into [error].
Replace the falsy fallback with an explicit isinstance(.., dict) guard at
both extra_content extraction sites (non-streaming normalize_response and
the streaming delta accumulator).
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