fix: cancel response bodies before retrying

[abhinavkr26104]

Summary

• release unused retryable response bodies before starting backoff
• support Web ReadableStream bodies and Node-style async iterable bodies
• keep bodyless retry responses working

Why

Retryable 429/5xx responses were abandoned without consuming or canceling their bodies. Streaming responses could retain sockets and reduce connection reuse across retries.

Testing

• ./scripts/test --runInBand (21 suites, 139 tests)
• ./scripts/build
• tsc --noEmit
• full ESLint run
• Prettier check for changed files

Fixes #204