[Bugfix][Rust Frontend] Restore prost test dependency

[khluu]

Summary

Restore prost as a vllm-server dev-dependency so its protobuf round-trip unit test compiles after the vllm-proto crate split.

Root cause

PR #56338 added a #[cfg(test)] import of prost::Message and calls to encode_to_vec() / decode(). Its exact CPU Rust Cargo and Cargo Style + Clippy jobs passed on the PR head in Buildkite #88199.

Before #56338 merged, PR #56365 moved protobuf generation into the new vllm-proto crate and removed prost from vllm-server. The stale-but-green #56338 head then merged as 1713a094, and main Buildkite #88858 failed deterministically in both CPU Rust Cargo jobs (and the equivalent MI250 jobs) with E0432/E0599.

This adds the dependency under [dev-dependencies], matching its test-only use, and updates the lockfile package dependency list.

Validation

• pre-commit run --files rust/src/server/Cargo.toml rust/Cargo.lock — passed
• git diff --check — passed
• Exact CPU Rust Cargo and Cargo Style + Clippy Buildkite validation is required on this draft; this host does not have a Rust toolchain installed.

Duplicate check

Searched open vLLM PRs for 56338, prost Rust server dependency, and Rust Frontend Cargo; no existing PR addresses this failure.

AI assistance

AI assistance was used to diagnose the cross-merge failure, prepare the minimal dependency fix, and run the listed checks. This is a draft for human line review and exact CI validation before readiness.

[khluu]

Exact validation is running in Buildkite #88872 at literal head 764b4bea9: the CPU Rust Frontend Cargo and Cargo Style + Clippy jobs both rendered and are running. This draft remains 🧪 until both pass and a human reviews the two-line diff.

[khluu]

Exact machine gate is green at literal head 764b4bea9: Buildkite #88872 passed terminally. CPU Rust Frontend Cargo passed in 3m28s, and Cargo Style + Clippy passed in 1m58s. Both directly reproduce the #88858 failing commands with the restored test-only dependency. The PR remains draft for human line review/readiness.

[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.