Studio: heal DiffusionGemma tool calls into structured tool_calls

[oobabooga]

DiffusionGemma tool calls come back as raw text instead of a structured tool_calls array. A client that declares a tool gets <|tool_call>call:get_weather{...}<tool_call|> in the message content with finish_reason: stop, so OpenAI-v1 clients like pi can't see the call.

Closes #6732

(Thinking already works; only tool calls were affected.)

Problem

supports_tools is off for diffusion, which keeps the agentic tool loop from running (it would drop the canvas frames that drive the visualization). But that same flag also switched off the client-tool passthrough, the path that turns the model's text tool call into a structured tool_calls.

So the tool call reached the client as raw text. The parser already understands DiffusionGemma's <|tool_call> format (from #6801); it just wasn't being reached.

Fix

Split the flag in two: supports_tools still gates the agentic loop (stays off for diffusion), and a new supports_tool_passthrough gates only the client-tool passthrough (on for diffusion).

Non-diffusion models are unaffected, since both flags return the same value there.

Verification

Live on unsloth/diffusiongemma-26B-A4B-it-GGUF with a declared tool: OpenAI (streaming and non-streaming) and Anthropic /v1/messages now return proper tool_calls / tool_use, and the diffusion visualization still streams. The existing passthrough and route suites pass unchanged.

Note

This is the Studio half. The model only emits schema-correct arguments once the shim (unslothai/unsloth-zoo#864) and the visual server (llama.cpp #24423) forward the tool definitions too, so it's a no-op until a client sends tools.

[gemini-code-assist]

Code Review

This pull request introduces a new property supports_tool_passthrough to the llama_cpp backend, which returns the underlying _supports_tools value. This property is then used in inference.py to determine tool passthrough and client tool support, falling back to supports_tools if the property is not present. This change ensures that client tool loops can bypass restrictions placed on supports_tools for specific models like DiffusionGemma. There are no review comments to address, and the changes look correct.

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[oobabooga]

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