Gemma emits tool calls as text — <|tool_call>call:NAME{…} — which the first-pass
streaming loop forwarded straight to the client as assistant content. So the call
showed up as a visible message and only got parsed/executed at end-of-stream.

Add _ToolCallStreamGate: as chunks stream, emit visible content only up to the
first tool-call marker (gemma's <|tool_call>/<|tool_response> plus the common tag
formats), holding back a small tail so a marker split across chunk boundaries
isn't leaked; once a marker appears, emit nothing further. The full text still
accumulates for the existing end-of-stream tool extraction (GemmaParser), which
emits a proper tool_calls delta. The second pass is already buffered+extracted, so
it never leaked.

Also add gemma's "<|tool_call>" to the reasoning/tool split pattern lists.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>