test: add unit tests for stripHtml and validateTextInput in sanitize.ts

[yachikadev]

Summary

Closes #2805

Added missing unit tests for stripHtml() and validateTextInput() functions in src/lib/sanitize.ts as per issue requirements.

Changes Made

Added the following missing test cases to test/sanitize.test.ts:

stripHtml()

• Plain text input returns unchanged output
• Apostrophe entity decoding (' and ')
• Mixed HTML tags and entities handled correctly

validateTextInput()

• Script tag injection is stripped and returns ok: true with clean value

Testing

All vitest tests pass locally.

[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

[yachikadev]

The Playwright smoke test failure is not related to this PR.
It is caused by a pnpm-lock.yaml lockfile mismatch with the current overrides config in package.json — a pre-existing repo issue. All checks relevant to this PR pass successfully.