Optimize Rust CI builds with proper sccache configuration

[slin1237]

• Add sccache to pr-test-pd-router.yml (was missing entirely)
• Add RUSTC_WRAPPER and SCCACHE_GHA_ENABLED env vars
• Replace old actions/cache with Swatinem/rust-cache + sccache
• Standardize shared-key: "rust-cache" across all jobs for cache reuse
• Add save-if: true to persist cache even on failure
• Add rust-cache to maturin-build-test job
• Add instructions for optional sccache setup
• Note that project defaults to incremental for local dev

This should significantly speed up Rust builds by:

1. Caching compiled crates across runs (rust-cache)
2. Caching individual compilation units (sccache with GHA backend)
3. Sharing cache between related jobs via shared-key

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
• Follow the SGLang code style guidance.
• Work with maintainers to merge your PR. See the PR Merge Process

[gemini-code-assist]

Summary of Changes

Hello @slin1237, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on enhancing the efficiency of Rust CI builds by implementing robust caching strategies. It introduces sccache and rust-cache to the CI workflows, aiming to drastically reduce build times by reusing compiled artifacts across runs and jobs. The changes also include updated documentation to guide developers on leveraging these caching mechanisms locally.

Highlights

• CI Build Optimization: Integrated sccache into pr-test-pd-router.yml and the maturin-build-test job to significantly speed up Rust CI builds.
• Caching Mechanism Update: Replaced the old actions/cache with Swatinem/rust-cache combined with sccache for more efficient caching of compiled crates and individual compilation units.
• Standardized Cache Configuration: Standardized the shared-key: "rust-cache" across all relevant jobs to maximize cache reuse and added save-if: true to persist caches even on build failures.
• Developer Documentation: Added a new 'Build Caching' section to sgl-model-gateway/README.md providing instructions for optional sccache setup for local development and clarifying its interaction with incremental compilation.

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Ignored Files

• Ignored by pattern: .github/workflows/** (3)
  • .github/workflows/pr-benchmark-rust.yml
  • .github/workflows/pr-test-pd-router.yml
  • .github/workflows/pr-test-rust.yml

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[gemini-code-assist]

Code Review

This pull request optimizes Rust CI builds by integrating sccache and updates the documentation to guide developers on using it locally. The changes in the README are clear and provide valuable instructions for build caching. I have one suggestion to improve the user-friendliness of the installation instructions by recommending package managers for a faster sccache setup.