Propagate client JSON-RPC errors for app-server request callbacks.
Previously a number of possible errors were collapsed to `channel
closed`. Now we should be able to see the underlying client error.
### Summary
This change stops masking client JSON-RPC error responses as generic
callback cancellation in app-server server->client request flows.
Previously, when the client responded with a JSON-RPC error, we removed
the callback entry but did not send anything to the waiting oneshot
receiver. Waiters then observed channel closure (for example, auth
refresh request canceled: channel closed), which hid the actual client
error.
Now, client JSON-RPC errors are forwarded through the callback channel
and handled explicitly by request consumers.
### User-visible behavior
- External auth refresh now surfaces real client JSON-RPC errors when
provided.
- True transport/callback-drop cases still report
canceled/channel-closed semantics.
### Example: client JSON-RPC error is now propagated (not masked as
"canceled")
When app-server asks the client to refresh ChatGPT auth tokens, it sends
a server->client JSON-RPC request like:
```json
{
"id": 42,
"method": "account/chatgptAuthTokens/refresh",
"params": {
"reason": "unauthorized",
"previousAccountId": "org-abc"
}
}
```
If the client cannot refresh and responds with a JSON-RPC error:
```
{
"id": 42,
"error": {
"code": -32000,
"message": "refresh failed",
"data": null
}
}
```
app-server now forwards that error through the callback path and
surfaces:
`auth refresh request failed: code=-32000 message=refresh failed`
Previously, this same case could be reported as:
`auth refresh request canceled: channel closed`
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