fix: guard against division by zero in cursor provider and deferred refresh race condition

[PolatOzaslan21]

Summary

This PR fixes two critical bugs identified during a code audit:

1. Division by zero in Cursor provider (src/providers/cursor.ts)

When includedApiUsd is 0, the expression usage.api.costUsd / includedApiUsd produces Infinity or NaN, resulting in -Infinity% or NaN% displayed in the quota toast.

Fix: Added a guard includedApiUsd > 0 ? ... : 0 to safely return 0 when the budget is zero.

Additionally, getEffectiveCursorIncludedApiUsd in src/lib/cursor-pricing.ts was rejecting 0 as a valid override (> 0 instead of >= 0). This meant users could not explicitly set a zero API budget. Changed to >= 0 so 0 is accepted as a valid override value.

2. Race condition in deferred quota refresh (src/plugin.ts)

In showQuotaToast, the finally block resets state.inFlight = false by looking up the state from the deferredQuotaRefreshes Map. However, during the try block, reconcileDeferredQuotaRefresh may clear the old state and schedule a new one — creating a new state object in the Map. The finally block would then find this new state and incorrectly set its inFlight to false, even though it was never set to true by the current invocation.

Fix: Added an identity check state === pendingDeferred in the finally block to ensure we only reset inFlight on the same state object that was marked at the start of the function.

Test

Added a test case in tests/providers.cursor.test.ts that verifies the cursor provider correctly handles cursorIncludedApiUsd: 0 without producing NaN or Infinity.

Changes

File	Change
src/providers/cursor.ts	Guard percentRemaining against division by zero
src/lib/cursor-pricing.ts	Accept 0 as valid overrideUsd (>= 0)
src/plugin.ts	Identity check in finally to prevent race condition
tests/providers.cursor.test.ts	New test for zero-budget scenario

Verification

• pnpm run typecheck passes
• pnpm run build passes
• pnpm test -- tests/providers.cursor.test.ts tests/lib.cursor-usage.test.ts — all 14 tests pass
• Tested against OpenCode Go (current production version)

[PolatOzaslan21]

Closes #127 — division by zero + race condition

Refs #128 — bun:sqlite compatibility issue (follow-up PR needed)