fix(client): use StreamableHTTPError instead of plain Error in send() (#1178)

Co-authored-by: Matt <[email protected]>

Author: yamadashy

src/client/streamableHttp.test.ts

@@ -209,7 +209,7 @@ describe('StreamableHTTPClientTransport', () => {
         const errorSpy = vi.fn();
         transport.onerror = errorSpy;
 
-        await expect(transport.send(message)).rejects.toThrow('Error POSTing to endpoint (HTTP 404)');
+        await expect(transport.send(message)).rejects.toThrow('Streamable HTTP error: Error POSTing to endpoint: Session not found');
         expect(errorSpy).toHaveBeenCalled();
     });
 

src/client/streamableHttp.ts

@@ -534,7 +534,7 @@ export class StreamableHTTPClientTransport implements Transport {
                     }
                 }
 
-                throw new Error(`Error POSTing to endpoint (HTTP ${response.status}): ${text}`);
+                throw new StreamableHTTPError(response.status, `Error POSTing to endpoint: ${text}`);
             }
 
             // Reset auth loop flag on successful response