Fix gemma4 parallel tool calling with MTP enabled

[Raf38]

Purpose

Fixes #41967.
Fix gemma4 tool parser when the model emits end tool and start tool in the same delta due to MTP speculative decoding. Without this fix the first tool call gets prematurely abandoned with out the proper json closures. The second tool call gets emitted correctly. I've added a new test case for the exact scenario we see.

Please can someone comment on this:
This PR removes the buffering of incoming deltas for the case where a start/end tool call token gets split across deltas. Given that Gemma4 has dedicated tokens for start and end of tool call we don't believe this is necessary. Across a few 10,000 tool call's we've never seen Gemma4-31B emit anything other than the special tokens. This buffering code appears to have been in the parser from the start leading us to believe it may have been a hangover from the model that inspired this parser?

Test Plan

Added new test case to pytest for the sequence of chunks that triggers the bug.
I had to modify test_streaming_does_not_duplicate_plain_text_after_tool_call as it was dubiously verifying the internal parser implementation rather than just the externally visible output.

Test Result

All tests pass.

There are a few other proposed PR's for this issue, but I believe this is the smallest in terms of minimal change to the existing implementation.

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[Raf38]

The issue (and test case I proposed here) has been fixed by #45163