[CI] Path-gate Rust workspace tests in lint

[JustinTong0323]

Summary

• detect changes that can affect the rust/ Cargo workspace
• run cargo test --workspace only for those changes
• keep the existing Rust cache and 900-second timeout unchanged

The relevant path set includes rust/**, the protobuf compiled by rust/sglang-grpc/build.rs, and the lint workflow itself. Changes to the gate therefore exercise the Cargo test path.

Motivation

PR #34860 changed only documentation, but its lint job still spent 1m44s in cargo test --workspace after an exact 390 MB Rust cache restore. The tests ran in 0.47s; test-profile compilation accounted for nearly all of the step. Documentation-only and unrelated Python changes cannot affect this workspace.

Validation

• SKIP=no-commit-to-branch pre-commit run --files .github/workflows/lint.yml
• YAML structure assertions for the path filter, conditional, timeout, and Cargo command
• dependency scan covering Cargo path dependencies and build-script inputs

This PR changes .github/workflows/lint.yml, so its own lint run is expected to execute the Rust workspace tests. After merge, a docs-only lint run should omit that step.

CI evidence

Lint run #31815484567 passed on this PR:

• changed-path detection: less than 1 second
• pre-commit: 6m41s
• Rust workspace tests: 1m45s
• total lint job: 10m57s

Because this PR changes the gate itself, the Rust workspace tests ran and passed. For an unrelated change, the new filter overhead is less than 1 second and the 1m45s Cargo step is skipped.

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CI States

Latest PR Test (Base): ✅ Run #31815485017
Latest PR Test (Extra): 🚫 Run #31816640159

[JustinTong0323]

/rerun-failed-ci

[JustinTong0323]

/rerun-failed-ci

[JustinTong0323]

/rerun-failed-ci