fix: avoid TypeError when timeout timer fires after test resolution (#90)

[mjn298]

Fixes #90.

Problem

Under event-loop contention, the TestRunner emits unhandled TypeError: Cannot read properties of undefined (reading 'reject') exceptions, failing CI even when every test passes.

Root cause

#createTimeoutTimer and #resetTimer install a setTimeout callback that dereferences this.#timeout!.reject at execution time rather than capture time. When Node dispatches a queued timer callback after the test has already resolved and #clearTimer() has set this.#timeout = undefined, the !. non-null assertion is wrong and the callback throws.

```ts
// src/test/runner.ts
timer: setTimeout(() => this.#timeout!.reject(this.#createError('Test timeout')), duration),
// ^^ #timeout may be undefined by the time this runs
```

Fix

Replace !. with ?. in both call sites. The callback safely becomes a no-op once #clearTimer has nulled the timeout — which is the correct behavior, since the outer promise has already resolved or rejected.

```diff

• timer: setTimeout(() => this.#timeout!.reject(this.#createError('Test timeout')), duration),

• timer: setTimeout(() => this.#timeout?.reject(this.#createError('Test timeout')), duration),
  ```

Two character changes total across #createTimeoutTimer and #resetTimer.

Test

Added a deterministic regression test in tests/test/execute.spec.ts. It stubs globalThis.setTimeout to capture the timer callback (so the timer never fires naturally), runs a test that resolves immediately, then invokes the captured callback after #clearTimer has run. Before the fix this throws the exact TypeError from the issue; after the fix it's a no-op.

Verified locally:

• Without the fix → new test fails with TypeError: Cannot read properties of undefined (reading 'reject')
• With the fix → full suite green (170/170)
• Lint clean

[thetutlage]

Thanks 👍🏽