fix(ext/web): fix infinite loop in WebTransport datagram overflow handling (#33075)
## Summary
- Fix overflow removal loop in `WebTransportDatagramDuplexStream` that
used a `const` counter never decremented, causing an infinite loop when
the incoming queue exceeded the high-water mark
- Change `const toBeRemoved` to `let` with a decrement so the loop
terminates correctly
- Add regression spec test that sends two datagrams without draining the
queue
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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>