spark-server: respect disabled MiniMax thinking

[lesserevil]

Summary

MiniMax M2/M2.7's native HF template always appends an open <think> at the generation prompt. This adds an OpenAI-variant minimax_m2 template that keeps explicit thinking unchanged, but emits a closed <think>\n\n</think>\n\n prompt when a request resolves to enable_thinking=false, so default and tool-active requests no longer get forced or leaked reasoning despite thinking_default=false.

Closes #

Test plan

• cargo fmt --all -- --check
• cargo test -p spark-server --lib tokenizer::tests -- --nocapture
• cargo test -p spark-server --lib tokenizer::tests::render_minimax_openai_template -- --nocapture
• ATLAS_SKIP_BUILD=1 CUDARC_CUDA_VERSION=13000 cargo clippy --workspace --tests -- -Dwarnings
• Deployed on EP=2 MiniMax M2.7 and verified Reply with exactly: OK returns reasoning_tokens=0
• ATLAS_SKIP_BUILD=1 cargo clippy --workspace --tests --all-features -- -Dwarnings (not runnable on Linux: enabling all features pulls in the Metal backend's Apple-only objc2 dependency, which compile-errors outside Apple targets)
• bash scripts/check-license-headers.sh (script is not present in this checkout; CI uses SkyWalking Eyes with .licenserc.yaml, and this PR does not add any SPDX-covered source file type)
• typos (not installed locally)
• Added or updated tests where applicable

Notes for reviewers

The native MiniMax template path remains unchanged. The fix uses the existing jinja-templates/openai/{model_type}.jinja lookup, mirroring the Qwen OpenAI-variant path. MiniMax needed a closed-thinking prompt, not simply no <think> prompt: with no tag, the model can still emit an implicit reasoning trace followed by </think>.

CLA

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