test : added unit tests for repo-analytics-utils and digest-email builders

[tmdeveloper007]

Closes #2628.

Summary of What Has Been Done:
Added two vitest test files covering untested modules in src/lib/. All 45 tests pass under vitest.

Changes Made:

• test/repo-analytics-utils.test.ts (20 tests): parseRepoParam for all valid/invalid GitHub repository identifier cases including boundary lengths, whitespace trimming, dot-only segments, and extra slashes.
• test/digest-email.test.ts (25 tests): buildDigestHtml and buildDigestText for all rendering states including metrics/no-metrics, empty language/repo sections, HTML escaping, and pluralisation.

Impact it Made:
repo-analytics-utils: parseRepoParam is the first gate in /api/metrics/repos and /api/metrics/repo-explorer. digest-email: the weekly digest email renders without crashing regardless of which metric fields are populated, and HTML entities in user data are properly escaped to prevent injection in email clients.

[github-actions]

GSSoC Label Checklist 🏷️

@Umbrella-io — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[Priyanshu-byte-coder]

CI is failing on this PR. Please fix the TypeScript/test errors before we can merge. Run npm run typecheck and npm test locally to reproduce.

[tmdeveloper007]

This PR has been closed as it has been stale for more than 2 days without recent activity or merging. If this work is still relevant, please reopen a fresh PR.