chore: update external connector deployment flow

app-instance/backend/beaver/interfaces/channels/connections/external.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import os
 from typing import Any
 
 from .models import ChannelRuntimeSpec, ValidationResult
@@ -37,6 +38,7 @@ class ExternalConnectorBase:
         self.credential_store = credential_store
         self.sidecar_client = sidecar_client
         self.sidecar_base_url = sidecar_base_url
+        self.callback_base_url = _callback_base_url()
 
     async def start_session(
         self,
@@ -63,7 +65,7 @@ class ExternalConnectorBase:
             "connectionId": connection.connection_id,
             "channelId": connection.channel_id,
             "displayName": connection.display_name,
-            "callbackBaseUrl": "",
+            "callbackBaseUrl": self.callback_base_url,
             "options": dict(options),
         }
         view = dict(await self.sidecar_client.start_session(payload))
@@ -176,6 +178,14 @@ def _policy_runtime_config(options: dict[str, Any]) -> dict[str, Any]:
     return result
 
 
+def _callback_base_url() -> str:
+    for name in ("EXTERNAL_CONNECTOR_CALLBACK_BASE_URL", "BEAVER_CONNECTOR_CALLBACK_BASE_URL"):
+        value = os.environ.get(name, "").strip()
+        if value:
+            return value.rstrip("/")
+    return ""
+
+
 def _string_list(value: Any) -> list[str]:
     if isinstance(value, str):
         raw_items = value.replace("\n", ",").split(",")