fix: fallback to app reviewer for rereview triggers

[keithah]

Summary

• keep team-based rereview requests as primary (aireview + alias fallback)
• when team requests fail, request the app reviewer as a fallback to keep @kodiai recheck effective
• add unit coverage for fallback reviewer request behavior

Verification

• bun test
• bunx tsc --noEmit

[Copilot]

Pull request overview

This PR improves the reliability of “UI-only” re-review triggers by falling back to requesting the GitHub App reviewer when the configured re-review team (and its alias) cannot be requested, ensuring @kodiai recheck can still generate an effective review_requested event.

Changes:

• Pass an app-slug-based fallbackReviewer into the best-effort re-review request helper from both review and mention handlers.
• Extend requestRereviewTeamBestEffort to request a fallback individual reviewer after team request attempts fail.
• Add unit coverage for the fallback reviewer request behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/handlers/review.ts	Supplies fallbackReviewer when auto-requesting the UI re-review team on PR open/ready events.
src/handlers/mention.ts	Supplies fallbackReviewer for @kodiai review / @kodiai recheck rereview triggers.
src/handlers/rereview-team.ts	Adds fallback reviewer request logic after team request failures.
src/handlers/rereview-team.test.ts	Adds a unit test verifying fallback reviewer is requested when team requests fail.

Comments suppressed due to low confidence (3)

src/handlers/mention.ts:29

• Unused import containsMention.

import {
  type MentionEvent,
  normalizeIssueComment,
  normalizeReviewComment,
  normalizeReviewBody,
  containsMention,
  stripMention,
} from "./mention-types.ts";

src/handlers/review.ts:524

• Unused variable appSlug.

            const appSlug = githubApp.getAppSlug();

src/handlers/mention.ts:138

• Unused variable possibleHandles.

    const possibleHandles = [appSlug, "claude"];

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[Copilot]

The fallback reviewer request doesn’t check whether that reviewer is already in listRequestedReviewers (users). If the team request fails and the fallback reviewer is already requested, GitHub typically returns 422 and no new review_requested event is emitted, which can make repeated recheck attempts ineffective and adds noisy warn logs. Consider extending the initial listRequestedReviewers check to also detect an already-requested fallbackReviewer (normalize/trim like elsewhere) and return alreadyRequested: true (or skip the fallback call) in that case.