Based on #11984 by @maxchernin. Fixes#8259.
Some providers (MiniMax M2.7 via NVIDIA NIM) resend the full function
name in every streaming chunk instead of only the first. The old
accumulator used += which concatenated them into 'read_fileread_file'.
Changed to simple assignment (=), matching the OpenAI Node SDK, LiteLLM,
and Vercel AI SDK patterns. Function names are atomic identifiers
delivered complete — no provider splits them across chunks, so
concatenation was never correct semantics.