test : added unit tests for upstash-rest.ts

[tmdeveloper007]

Closes #2830.

Summary of What Has Been Done: Added vitest unit tests for the Upstash Redis REST helpers in src/lib/upstash-rest.ts, covering getUpstashConfig, upstashPipeline, upstashRateLimitFixedWindow, and upstashTryAcquireLock.

Changes Made:

• Test getUpstashConfig: null when missing env vars, returns config when both present
• Test upstashPipeline: empty array when not configured, empty on fetch failure, parsed JSON on success
• Test upstashRateLimitFixedWindow: allowed when not configured or fetch fails, allowed when count is NaN, blocked when over limit
• Test upstashTryAcquireLock: false when not configured, true when SET returns OK, false when SET returns null

Impact it Made: Upstash Redis powers distributed rate limiting and locking. Test coverage validates behavior across all configuration and response scenarios.

[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