test : added unit tests for digest-email.ts

[tmdeveloper007]

Closes #2860.

Summary of What Has Been Done:
Added a comprehensive test suite for the digest-email.ts utility module covering esc, pluralise, langColour, buildDigestHtml, and buildDigestText functions.

Changes Made:

• Created test/digest-email.test.ts with 19 test cases
• Tested esc function with HTML special characters
• Tested pluralise with singular and plural forms
• Tested langColour with known and unknown language names
• Tested buildDigestHtml with greeting, weekLabel, unsubscribe, streak, activity, languages, repos, and HTML escaping
• Tested buildDigestText with greeting, streak with emoji, activity, language percentages, and repo listing

Impact it Made:
Increases test coverage for the lib directory and ensures email template generation handles all edge cases correctly.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — 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