The WT path duplicated upstream resolution, LB selection, header ops,
replacer vars, and in-flight counters. Route WT through the shared
ServeHTTP -> proxyLoopIteration -> reverseProxy flow and swap RoundTrip
for a small webTransportHijack that only does WT-specific work (writer
unwrap, upstream dial, client upgrade, pump).
Rename roundtripSucceededError -> terminalError. The existing name
described when it was emitted (after a successful round-trip); the
new name describes its contract with the retry loop (stop looping,
propagate error unchanged). The WebTransport upgrade case is a second
natural caller for that same signal.
Comes with two behavior improvements that fall out of the collapse:
- WT upstream dial failures now surface as DialError, so the loop
can fail over across upstreams like normal proxies (today: 502).
- Passive health checks apply to WT dials (dial-failure countFailure
and UnhealthyLatency on dial duration) via the shared path.
Addresses reviewer feedback that the duplicated setup was a
maintenance risk.
2d60b81d75