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.

coderd/apidoc/docs.go

@@ -20837,6 +20837,14 @@ const docTemplate = `{
                         }
                     ]
                 },
+                "subagent_id": {
+                    "format": "uuid",
+                    "allOf": [
+                        {
+                            "$ref": "#/definitions/uuid.NullUUID"
+                        }
+                    ]
+                },
                 "workspace_folder": {
                     "type": "string"
                 }

coderd/apidoc/swagger.json

@@ -19141,6 +19141,14 @@
 						}
 					]
 				},
+				"subagent_id": {
+					"format": "uuid",
+					"allOf": [
+						{
+							"$ref": "#/definitions/uuid.NullUUID"
+						}
+					]
+				},
 				"workspace_folder": {
 					"type": "string"
 				}