rust-guard: extract first_matching_label helper and add label-check unit tests

[Copilot]

first_matching_approval_label and first_matching_refusal_label were near-identical — differing only in which PolicyContext field they accessed. Adds a shared private helper and direct unit tests for all four label-check functions.

Refactor: extract first_matching_label

Replaces the duplicated body in both functions with a single helper that takes the label list directly:

fn first_matching_label<'a>(item: &'a Value, label_list: &[String]) -> Option<&'a str> {
    if label_list.is_empty() {
        return None;
    }
    extract_github_label_names(item)
        .into_iter()
        .find(|name| label_list.iter().any(|l| l.eq_ignore_ascii_case(name)))
}

fn first_matching_approval_label<'a>(item: &'a Value, ctx: &PolicyContext) -> Option<&'a str> {
    first_matching_label(item, &ctx.approval_labels)
}

fn first_matching_refusal_label<'a>(item: &'a Value, ctx: &PolicyContext) -> Option<&'a str> {
    first_matching_label(item, &ctx.refusal_labels)
}

Tests: direct unit tests for label-check helpers in helpers.rs

Adds 10 tests covering has_approval_label, has_refusal_label, has_promotion_label, and has_demotion_label:

• Case-insensitive matching
• Empty list / empty string → feature disabled (false)
• Missing labels field on the item → false

[Copilot]

Pull request overview

This PR refactors the rust-guard label-matching logic by extracting duplicated approval/refusal label search into a shared helper, and adds targeted unit tests for the label-check helpers to validate behavior like case-insensitive matching and “feature disabled” defaults.

Changes:

• Extracted a shared first_matching_label helper used by both first_matching_approval_label and first_matching_refusal_label.
• Added unit tests for has_approval_label, has_refusal_label, has_promotion_label, and has_demotion_label covering key edge cases (case-insensitivity, disabled config, missing labels).

Show a summary per file

File	Description
guards/github-guard/rust-guard/src/labels/helpers.rs	Deduplicates approval/refusal label matching via a shared helper and adds direct unit tests for label-checking helpers.

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