rust-guard: flatten commit integrity branch logic and expand trusted bot variant coverage

[Copilot]

This updates Rust guard labeling in two low-risk areas: it removes duplicated branching in get_commit/list_commits integrity selection, and adds explicit coverage for all trusted first-party bot username variants (including app/ forms and merge queue bot).

• Commit integrity rule clarity (tool_rules.rs)

  • Replaced a double-nested conditional with an equivalent 3-way branch.
  • Behavior remains the same, but the policy is now explicit: default refs always map to merged_integrity; non-default refs in private repos map to writer_integrity; otherwise empty integrity.

• Trusted bot coverage completeness (helpers.rs)

  • Added a focused unit test for is_trusted_first_party_bot covering all canonical entries in TRUSTED_FIRST_PARTY_BOTS.
  • Includes case-insensitivity checks and negative assertions for non-trusted usernames.

• Illustrative snippet (logic simplification)

integrity = if is_default_ref {
    merged_integrity(repo_id, ctx)
} else if repo_private_effective {
    writer_integrity(repo_id, ctx)
} else {
    vec![]
};

[Copilot]

Pull request overview

This PR updates the GitHub Rust guard’s labeling logic to make commit-integrity selection clearer while keeping behavior unchanged, and adds unit test coverage to ensure trusted first-party bot username variants are recognized consistently.

Changes:

• Simplified the get_commit / list_commits integrity selection into an explicit 3-way branch (default ref → merged integrity; private non-default → writer integrity; otherwise empty integrity).
• Added a unit test exercising trusted first-party bot username variants (including app/ forms) and case-insensitivity.

Show a summary per file

File	Description
guards/github-guard/rust-guard/src/labels/tool_rules.rs	Refactors integrity-branching logic for commit-related tools into a clearer equivalent conditional structure.
guards/github-guard/rust-guard/src/labels/helpers.rs	Adds a targeted unit test for trusted first-party bot username variant recognition.

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• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Low