## Summary
Fixes cross-replica chat relay failing with:
```
failed to open initial relay for chat stream
error= dial relay stream: - failed to WebSocket dial: expected handshake response status code 101 but got 200
failed to open relay for message parts
error= dial relay stream: - failed to WebSocket dial: expected handshake response status code 101 but got 200
```
Subscribers see accurate `status=running` (delivered via pubsub) but
miss all in-progress `message_part` events (delivered only via the relay
WebSocket that never connects).
## Root cause
`redirectToAccessURL` in `cli/server.go` redirects any request whose
`Host` header doesn't match the access URL. The enterprise chat relay
dials another replica directly via its DERP relay address (e.g.
`http://10.0.0.2:8080`), so the `Host` header is the pod IP — not the
access URL.
This triggers a **307 redirect** to the access URL. The WebSocket
library follows the redirect, but the second request is a plain GET —
`Connection: Upgrade` and `Upgrade: websocket` headers are **not carried
over** by HTTP redirect semantics. The load-balanced access URL routes
the plain GET to any replica, which serves the SPA catch-all handler and
returns **HTTP 200 with `index.html`**.
The WebSocket library then fails: `expected handshake response status
code 101 but got 200`.
DERP mesh already has an exemption for this exact scenario
(`isDERPPath`). Chat relay was added later and didn't get one.
## Fix
Bypass `redirectToAccessURL` for requests that carry the
`X-Coder-Relay-Source-Replica` header, which the enterprise relay
already sets on every request (`enterprise/coderd/chatd/chatd.go:573`).
## Sequence diagram
**Before (broken):**
```
Replica A (subscriber) Replica B (worker) Load Balancer
| | |
|--- WS dial pod-ip:8080 ----->| |
| |-- 307 redirect to LB --->|
| | |
|<----------- plain GET (no Upgrade headers) ------------->|
| | |-- routes to any replica
|<----------- 200 index.html -------------------------------|
| |
X 'expected 101 but got 200' |
```
**After (fixed):**
```
Replica A (subscriber) Replica B (worker)
| |
|--- WS dial pod-ip:8080 ----->|
| (X-Coder-Relay-Source- |
| Replica header set) |
| |-- bypass redirect
|<--------- 101 Upgrade ------|
|<==== message_part events ====|
```
Kyle Carberry
a6b9a25f82