fix: [#2339] Prevent unhandled rejection in Animation.cancel()

[CNSeniorious000]

Description

Animation.cancel() can fail tests with an unhandled AbortError even when finished is never read. Following the cancellation algorithm, mark the promise handled, replace it on cancellation, and reuse the replacement on idle-to-play while preserving the old promise's rejection.

Resolves #2339

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CI passed on Node 20, 22, and 24.

Local validation: 205 Animation/Element tests, TypeScript compilation, ESLint, and Prettier passed. Earlier Chromium 148 checks confirmed promise identity and cancellation behavior. The earlier full local run had 9 macOS canvas snapshot mismatches; the current full compile attempt hit a chromium.googlesource.com download timeout.

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