test : added unit tests for supabase-browser env validation

[tmdeveloper007]

Closes #2814.

Summary of What Has Been Done: Created test/supabase-browser.test.ts with 7 test cases covering isBrowserClientAvailable across all env-var combinations: valid config, placeholder URL, placeholder key, missing vars, and empty strings.

Changes Made:

• test/supabase-browser.test.ts: 7 test cases using vi.resetModules() and process.env mocking to test the module-level isBrowserClientAvailable boolean.

Impact it Made: isBrowserClientAvailable determines whether the browser-safe Supabase client is usable in the current environment. Tests protect against regressions when the env validation logic changes.

[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