RunsClient.create: support stateless Run creation (#1694)

Co-authored-by: David Duong <david@duong.cz>

.changeset/thirty-boats-hear.md

@@ -0,0 +1,5 @@
+---
+"@langchain/langgraph-sdk": patch
+---
+
+Add support for creating stateless runs

libs/sdk/src/client.ts

@@ -1178,7 +1178,7 @@ export class RunsClient<
    * @returns The created run.
    */
   async create(
-    threadId: string,
+    threadId: string | null,
     assistantId: string,
     payload?: RunsCreatePayload
   ): Promise<Run> {
@@ -1210,7 +1210,8 @@ export class RunsClient<
         : undefined,
     };
 
-    const [run, response] = await this.fetch<Run>(`/threads/${threadId}/runs`, {
+    const endpoint = threadId === null ? "/runs" : `/threads/${threadId}/runs`;
+    const [run, response] = await this.fetch<Run>(endpoint, {
       method: "POST",
       json,
       signal: payload?.signal,