[Rust Frontend][gRPC] Preserve skip_special_tokens decoding option

[biswapanda]

Purpose

Align the native Generate gRPC API with vLLM's existing Python serving APIs by carrying the per-request skip_special_tokens decoding option through ResponseOptions.

The Python chat completions, completions, responses, and token-native APIs default this option to true, but the gRPC request did not expose it. Consequently, a gRPC caller could not explicitly preserve tokenizer-defined special markers required by some reasoning and tool parsers.

The protobuf field is optional so the server can distinguish omission from an explicit false. Omitted values retain the established Python default of true; explicit false values are preserved. This affects decoded text only and does not change sampling, generated token IDs, or EOS behavior.

• Python API behavior

Endpoint	Field location	Default
/v1/chat/completions	Top-level request field	true
/v1/completions	Top-level request field	true
/v1/responses	Top-level request field	true
/inference/v1/generate	sampling_params.skip_special_tokens	true

Test Plan

• Verify Rust formatting.
• Run the existing gRPC conversion test module after mapping the new response option into TextDecodeOptions.

Test Result

$ cargo fmt --all -- --check

$ cargo test -p vllm-server grpc::convert::tests
running 13 tests
test result: ok. 13 passed; 0 failed; 0 ignored; 0 measured; 328 filtered out

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[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[njhill]

Thanks @biswapanda

[njhill]

/ci run

[github-actions]

✅ @biswapanda, CI is now available for this PR.

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[github-actions]

✅ Triggered Buildkite CI #83993 for commit c98c11b154af.