ci: lock Rust checks and test prefill feature

[afourniernv]

What

• Run the existing Rust Clippy and test commands with --locked.
• Add focused Clippy and test coverage for the optional prefill-router integration.

Why

The current Rust job does not fail when the Cargo manifests and committed lockfile disagree.

Cargo documents --locked as requiring the dependency versions recorded in Cargo.lock to remain unchanged. It exits with an error if the lockfile is missing or dependency resolution would update it.

It also tests the standalone prefill-router workspace crate without enabling the optional integration in switchyard-runner or the feature forwarded by switchyard-server.

The existing default checks stay in place because they compile the feature-disabled runner path. The two new steps cover the enabled path without rerunning every workspace test with --all-features.

Notes for reviewers

This changes one workflow file (+6/-2). It does not change runtime behavior, public APIs, dependencies, packages, or release workflows.

How tested

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo clippy -p switchyard-server --all-targets --features prefill-router --locked -- -D warnings
• cargo test --workspace --locked
• cargo test -p switchyard-runner --features prefill-router --locked
• git diff --check

No live provider test was run because this only changes the Cargo commands used by CI.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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📥 Commits

Reviewing files that changed from the base of the PR and between cb98af0 and b1e1c79.

📒 Files selected for processing (1)

• .github/workflows/ci.yml

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Walkthrough

The Rust CI workflow now uses locked dependencies for workspace checks and adds Clippy and test coverage for the prefill-router feature.

Changes

Rust CI validation

Layer / File(s)	Summary
Rust CI command coverage .github/workflows/ci.yml	Workspace Clippy and tests now use --locked. Dedicated prefill-router checks cover switchyard-server and switchyard-runner.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to b1e1c

This change strengthens Rust CI validation without altering runtime behavior or public APIs. No actionable merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely summarizes the CI changes: locked Rust checks and added testing for the prefill-router feature.

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_{A rabbit checks the Rusty gate
Locked crates wait in tidy state
Clippy hops through router trails
Tests twitch their little tails
Green leaves bloom in CI’s wake}

_{Comment @coderabbitai help to get the list of available commands.}

[ayushag-nv]

@grahamking for review