chore(coderd): introduce TaskAppID and deprecate AITaskSidebarAppID (#20336)

As we're moving away from the SidebarAppID nomenclature, this PR
introduces a new `TaskAppID` field to `codersdk.WorkspaceBuild` and
deprecates the `AITaskSidebarAppID` field. They both contain the same
value.

coderd/apidoc/docs.go

@@ -20462,6 +20462,7 @@ const docTemplate = `{
             "type": "object",
             "properties": {
                 "ai_task_sidebar_app_id": {
+                    "description": "Deprecated: This field has been replaced with ` + "`" + `TaskAppID` + "`" + `",
                     "type": "string",
                     "format": "uuid"
                 },
@@ -20543,6 +20544,10 @@ const docTemplate = `{
                         }
                     ]
                 },
+                "task_app_id": {
+                    "type": "string",
+                    "format": "uuid"
+                },
                 "template_version_id": {
                     "type": "string",
                     "format": "uuid"