fix(ci): retry whisper model downloads

[kentcdodds]

Summary

• Add retry/backoff handling for whisper.cpp model downloads, including 429 responses from Hugging Face.
• Retry interrupted response body reads as well as retryable HTTP/network failures.
• Add unit coverage for rate-limit retries, interrupted downloads, and permanent failures.
• Narrow the injectable fetch type so test doubles do not need Bun-specific fetch properties.

Testing

• bun test src/whispercpp-transcribe.test.ts
• bun run typecheck
• bun run validate
• GitHub PR checks: validate test/main jobs passed, playwright-smoke passed, e2e passed, Bugbot passed; release job skipped as expected.