fix(api): type initial_message_status as a Literal enum

str | None under-specified the field: the implementation and the TS SDK
union both constrain it to queue_full / refused_closed, and the Literal
projects a proper enum into the generated OpenAPI spec so clients
reject unexpected values. Specs regenerated.

sdk/typescript/openapi-server.json

@@ -2568,6 +2568,10 @@
           "initial_message_status": {
             "anyOf": [
               {
+                "enum": [
+                  "queue_full",
+                  "refused_closed"
+                ],
                 "type": "string"
               },
               {

turnstone/api/server_schemas.py

@@ -224,7 +224,7 @@ class CreateWorkstreamResponse(BaseModel):
             "/v1/api/workstreams/{ws_id}/send."
         ),
     )
-    initial_message_status: str | None = Field(
+    initial_message_status: Literal["queue_full", "refused_closed"] | None = Field(
         default=None,
         description=(
             "Present ONLY when the workstream was created but its "