fix(chat_format): parse Gemma 4 native tool-call tokens into tool_calls (#2227)

[Anai-Guo]

Summary

Closes #2227.

Adds @register_chat_completion_handler("gemma4") so that create_chat_completion() with Gemma 4 + tools actually returns parsed tool_calls instead of dumping native tokens into message.content.

What changes

• llama_cpp/llama_chat_format.py

  • New _parse_gemma4_native_tool_calls(text) — pure-Python parser for the Gemma 4 native tool-call grammar, including the optional <|channel>thought…<channel|> block that thinking mode adds.
  • New gemma4_chat_completion handler that uses the GGUF-embedded Jinja2 chat template for prompt rendering, runs llama.create_completion, and post-parses the output.
  • Adds import re.

• tests/test_llama_chat_format.py — 8 new tests covering the issue repro, mixed primitives (int/float/bool/null), list of strings, thought-block stripping, plain-text passthrough, multiple sequential calls, surrounding plain text, and string values with embedded ".

Why this design

• Reuse the GGUF Jinja template. Gemma 4 GGUFs already ship a correct chat template that produces the right tool-prompt tokens — the bug was strictly on the parsing side, not the formatting side. Re-using Jinja2ChatFormatter keeps prompt rendering in lockstep with whatever the model author shipped, instead of hard-coding another copy that can drift.

• Match the C++ side. ggml-org/llama.cpp#21326 already added the equivalent PEG parser to llama-server. This PR is the Python port, with the same grammar:

  Type	Encoding
  string	key:<|"|>value<|"|>
  int	key:30
  float	key:3.5
  bool	key:true / key:false
  null	key:null
  list	key:[v1,v2,...]

  The 3-char <|"|> delimiter means a literal " inside a string value never terminates it — no escape handling needed.

Known limitation

Streaming responses currently pass chunks through unchanged; the caller still gets the raw native tokens. A streaming tool-call parser needs the same incremental PEG state machine the C++ side uses, which is a bigger change. The public _parse_gemma4_native_tool_calls helper is documented so callers can buffer chunks and re-parse if they need streaming today.

Test plan

• Pure-parser tests pass locally (no GGUF download required for the new tests — they exercise _parse_gemma4_native_tool_calls directly, matching the style of the existing tests in this file).
• Maintainer-side: end-to-end with a real gemma-4-*.gguf and a tools request, to confirm the Jinja-template path renders correctly and the handler returns tool_calls.

References

• Issue: Gemma 4 tool calls returned as raw native tokens in content instead of tool_calls #2227
• Gemma 4 prompt format spec: https://ai.google.dev/gemma/docs/core/prompt-formatting-gemma4
• C++ port: Gemma 4 template parser fixes ggml-org/llama.cpp#21326
• Related upstream issues: Misc. bug: Gemma4 tool calling leaves unexpected tokens in tool calls ggml-org/llama.cpp#21316, Eval bug: Gemma 4 tool call returned as content ggml-org/llama.cpp#22786

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[Anai-Guo]

Closing this PR. After fixing the initial ruff E402 violations, ruff format --check flagged additional formatting drift in both llama_cpp/llama_chat_format.py and tests/test_llama_chat_format.py that I can't cleanly resolve without running the formatter locally. This PR was opened by an automated pipeline under my account without me catching it; rather than push a half-fixed branch I'd rather hand the slate back. Issue #2227 remains a real bug — the C++ llama-server PEG-grammar parser (ggml-org/llama.cpp#21326) is the reference fix. Apologies for the noise.

[Anai-Guo]

Withdrawing orphan PR; #2227 remains open for a clean re-attempt.

[Anai-Guo]

Reopening after resolving the ruff format --check drift that I'd flagged when withdrawing this earlier. The two files now pass both ruff check llama_cpp tests and ruff format --check llama_cpp tests locally (ruff 0.15.14, matching the CI pin >=0.15.7), and the new gemma4 parser tests pass. No logic changed since the original review — only formatting.

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