test: add unit tests for pr-volume-bucketing (re #885)

[Ishaan-Gpt]

Summary

Adds unit tests for the PR volume bucketing helper in src/lib/maintainer/pr-volume-bucketing.ts, covering one file from the #885 checklist (one test file per PR).

Type of Change

• Bug fix
• New feature
• UI / UX improvement
• Refactor
• Documentation
• Other (unit tests)

Related Issue

Related to #885

What was changed?

New file src/lib/maintainer/pr-volume-bucketing.test.ts (16 tests):

• Bucket layout: one bucket per day for 7d/30d, weekly buckets for 90d, monthly buckets with "Mon YYYY" labels for all; empty input starts every counter at zero; the function never mutates its date arguments.
• Merged counts: a merged PR lands in the bucket containing its mergedAt; merges outside the window (before from, at or after to) are ignored; weekly and monthly bucketing place merges correctly.
• AI blocked counts: a flagged PR counts toward the bucket containing its creation date; unflagged PRs are skipped.
• Stalled counts: a PR goes stale 14 days after its last update and is counted in every bucket while it stays open; it drops out of the stalled count after being closed; a PR closed before the window is never counted.

Screenshots

Not applicable (no UI changes).

Checklist

• My code follows the project structure and conventions
• I tested this locally (npx vitest run src/lib/maintainer/pr-volume-bucketing.test.ts — 16/16 pass)
• No hardcoded secrets or credentials
• I have updated documentation if needed
• npx prettier --check passes on the new file; tsc --noEmit shows no new errors

[github-actions]

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