feat(agent): populate subagent ID for terraform-defined devcontainers (#21942)

Completes the final piece of the puzzle. Support the pre-creation flow
from the agent side.

docs/reference/api/agents.md

@@ -838,6 +838,10 @@ curl -X GET http://coder-server:8080/api/v2/workspaceagents/{workspaceagent}/con
       "id": "497f6eca-6276-4993-bfeb-53cbbbba6f08",
       "name": "string",
       "status": "running",
+      "subagent_id": {
+        "uuid": "string",
+        "valid": true
+      },
       "workspace_folder": "string"
     }
   ],
@@ -1015,6 +1019,10 @@ curl -X GET http://coder-server:8080/api/v2/workspaceagents/{workspaceagent}/con
       "id": "497f6eca-6276-4993-bfeb-53cbbbba6f08",
       "name": "string",
       "status": "running",
+      "subagent_id": {
+        "uuid": "string",
+        "valid": true
+      },
       "workspace_folder": "string"
     }
   ],