fix: change pid return type from undefined to be null

src/client/stdio.test.ts

@@ -64,7 +64,7 @@ test("should return child process pid", async () => {
   const client = new StdioClientTransport(serverParameters);
 
   await client.start();
-  expect(client.pid).toBeDefined();
+  expect(client.pid).not.toBeNull();
   await client.close();
-  expect(client.pid).toBeUndefined();
+  expect(client.pid).toBeNull();
 });

src/client/stdio.ts

@@ -184,8 +184,13 @@ export class StdioClientTransport implements Transport {
     return this._process?.stderr ?? null;
   }
 
-  get pid(): number | undefined {
-    return this._process?.pid;
+  /**
+   * The child process pid spawned by this transport.
+   *
+   * This is only available after the transport has been started.
+   */
+  get pid(): number | null {
+    return this._process?.pid ?? null;
   }
 
   private processReadBuffer() {