spark-server: respect MiniMax auto tool choice

[lesserevil]

Summary

MiniMax chat requests now resolve an effective tool mode before sampling so tool_choice: "auto" remains optional instead of forcing <tool_call> generation. Loop suppression also disables tool-call grammar for one recovery turn unless the caller explicitly requires a tool call.

Test plan

• cargo fmt --all -- --check
• LIBRARY_PATH=/opt/vllm/nccl-blackwell/lib LD_LIBRARY_PATH=/opt/vllm/nccl-blackwell/lib ATLAS_SKIP_BUILD=1 cargo test -p spark-server sampling_tool_mode -- --nocapture
• LIBRARY_PATH=/opt/vllm/nccl-blackwell/lib LD_LIBRARY_PATH=/opt/vllm/nccl-blackwell/lib ATLAS_SKIP_BUILD=1 CUDARC_CUDA_VERSION=13000 cargo clippy -p spark-server --tests -- -Dwarnings
• ATLAS_SKIP_BUILD=1 cargo clippy --workspace --tests --all-features -- -Dwarnings could not complete on this Linux host because the workspace hits the pre-existing objc2 Apple-platform compile gate before this change.
• bash scripts/check-license-headers.sh could not run because scripts/check-license-headers.sh is not present in this checkout.
• typos could not run because typos is not installed on this host.
• Tested against a real model / hardware: deployed on godspeed + savitar with nvidia/MiniMax-M2.7-NVFP4 EP=2; Hermes hello! returned a normal assistant response with no repeated recall/tool loop, and a direct synthetic loop-suppression request returned assistant text while logging tool-call grammar suppression.
• Added focused unit coverage for MiniMax auto/required/specific tool choices and loop suppression.

Notes for reviewers

This keeps bare-JSON tool mode required, but MiniMax XML tools now follow normal OpenAI auto semantics. Positive <tool_call> logit bias is only applied when a tool call is required, and loop suppression now hard-masks tool-call start tokens because explicit required/specific requests are filtered out before reaching the scheduler.

Benchmarks: not run; this is a tool-choice correctness fix, not a performance-oriented change.

Authorship: AI-generated by Codex under human operator direction; no human-written code sections are claimed.

CLA

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