test(pendingAttempt): cover the guest exam-save persistence and flush

[nastaso]

Context

src/lib/pendingAttempt.ts is the guest mock-exam save path: readPendingAttempt validates the payload shape and expires entries past a 24h TTL, and flushPendingAttempt (line 122) mirrors the live save with a compensating delete on a partial write (deletes the exam_attempts row if the attempt_questions insert fails) plus a concurrency guard. All of it is untested, and a regression could silently drop a guest's completed attempt or leave a scored attempt with zero reviewable questions.

Scope

• Add src/lib/pendingAttempt.test.ts (co-located, Vitest). Reuse the mock-Supabase harness pattern from src/lib/supabaseUtils.test.ts. Tests only; do not change production behavior.

Cases to cover

• storePendingAttempt then readPendingAttempt round-trips a valid payload.
• readPendingAttempt returns null for an expired entry (past the TTL) and for a malformed / shape-invalid entry.
• flushPendingAttempt: success path inserts both rows; on an attempt_questions failure it deletes the exam_attempts row (compensating rollback); the concurrency guard prevents a double flush.

Acceptance criteria

• src/lib/pendingAttempt.test.ts covers round-trip, TTL expiry, shape rejection, partial-write rollback, and the concurrency guard.
• npm run test passes; no new dependencies; no production code changed.

[nastaso]

Overtaken by #117/#118: src/lib/pendingAttempt.test.ts now exists and covers the save-intent flush path, and the API moved (peekPendingAttempt is the public wrapper; flushPendingAttempt(userId)). The task as written would collide with current code. Closing; I will re-file a tight follow-up if the flush-rollback branch is worth pinning.