spark-server: avoid orphan tool messages in F32

[lesserevil]

Summary

F32 no longer clones an old failed role: tool message onto the end of the conversation. It now surfaces the failed tool result in a runtime reminder, preserving OpenAI tool-message ordering so MiniMax and other vendor templates do not reject the prompt as an orphan tool result.

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 f32 -- --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 still fails on this Linux host before this patch because the workspace pulls objc2, which requires an Apple target.
• 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; a synthetic F32 regression request now returns HTTP 200 and logs F32: surfaced most-recent failed tool_result in a runtime reminder; Hermes smoke test no longer hits the MiniMax template 400.
• Added focused unit coverage for stale failed-tool surfacing and the already-fresh no-op case.

Notes for reviewers

The previous F32 behavior attempted to make a stale failure fresh by duplicating the original tool result. That creates invalid OpenAI history whenever messages after the original assistant/tool pair are not assistant tool calls, and MiniMax's template rejects it with "Message has tool role, but there was no previous assistant message with a tool call".

This keeps the guard's intent while staying inside the template contract: the stale failure is copied into a system-reminder appended to the latest user/tool message instead of being represented as a new role: tool turn.

Benchmarks: not run; this is a prompt-history 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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