Isolate Node reasoning model fixture

Use a fresh client for the streaming reasoning test so its gpt-5.4 model catalog is not shadowed by the shared client's earlier cached fixture.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 9b80f6cb-b851-4598-8294-f4431c6f3143

Author: ellismg

nodejs/test/e2e/streaming_fidelity.e2e.test.ts

@@ -146,7 +146,9 @@ describe("Streaming Fidelity", async () => {
     });
 
     it("should emit streaming deltas with reasoning effort configured", async () => {
-        const session = await client.createSession({
+        const reasoningClient = createClient();
+        onTestFinished(() => reasoningClient.stop());
+        const session = await reasoningClient.createSession({
             onPermissionRequest: approveAll,
             model: "gpt-5.4",
             streaming: true,