fix: ensure websocket close messages are truncated to 123 bytes (#779)

It's possible for websocket close messages to be too long, which cause
them to silently fail without a proper close message. See error below:

```
2022-03-31 17:08:34.862 [INFO]	(stdlib)	<close_notjs.go:72>	"2022/03/31 17:08:34 websocket: failed to marshal close frame: reason string max is 123 but got \"insert provisioner daemon:Cannot encode []database.ProvisionerType into oid 19098 - []database.ProvisionerType must implement Encoder or be converted to a string\" with length 161"
```

coderd/httpapi/httpapi.go

@@ -115,3 +115,21 @@ func Read(rw http.ResponseWriter, r *http.Request, value interface{}) bool {
 	}
 	return true
 }
+
+const websocketCloseMaxLen = 123
+
+// WebsocketCloseSprintf formats a websocket close message and ensures it is
+// truncated to the maximum allowed length.
+func WebsocketCloseSprintf(format string, vars ...any) string {
+	msg := fmt.Sprintf(format, vars...)
+
+	// Cap msg length at 123 bytes. nhooyr/websocket only allows close messages
+	// of this length.
+	if len(msg) > websocketCloseMaxLen {
+		// Trim the string to 123 bytes. If we accidentally cut in the middle of
+		// a UTF-8 character, remove it from the string.
+		return strings.ToValidUTF8(string(msg[123]), "")
+	}
+
+	return msg
+}