test(server-utils): Make the trace continuation assertions provable

Both cases passed vacuously. `not.toBe(TRACE_ID)` on the malformed carrier also
held when no agent span was opened at all, since `undefined` is not the carrier's
id either; suppressing span creation entirely failed nine other tests and left
that one green. And the active-trace case never checked the span landed on the
surrounding request's trace, so continuing an unrelated new trace passed too.

Now the first asserts a well-formed trace id, and the second asserts the agent
span carries the `incoming request` span's own trace id. Dropping the
`!getActiveSpan()` guard fails the second.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: RulaKhaled

packages/server-utils/test/ai/lib/tracing/flue.test.ts

@@ -202,17 +202,23 @@ describe('createFlueInstrumentation', () => {
         async () => undefined,
       );
 
+      // Asserting a well-formed id, not just "not the carrier's": the span has to still be opened
+      // on a fresh trace, and `not.toBe` alone would also pass if no span were opened at all.
+      expect(agentTraceId()).toMatch(/^[0-9a-f]{32}$/);
       expect(agentTraceId()).not.toBe(TRACE_ID);
     });
 
     // An in-process dispatch is genuinely part of the surrounding trace; continuing the persisted
     // one would detach it from the request it is actually running inside.
     it('keeps the active trace when one is already running', async () => {
-      await startSpan({ name: 'incoming request' }, async () => {
+      let requestTraceId: string | undefined;
+
+      await startSpan({ name: 'incoming request' }, async span => {
+        requestTraceId = spanToStaticSpanJSON(span).trace_id;
         await instrumentation.interceptor(AGENT_OP, { ...AGENT_CTX, traceCarrier: CARRIER }, async () => undefined);
       });
 
-      expect(agentTraceId()).not.toBe(TRACE_ID);
+      expect(agentTraceId()).toBe(requestTraceId);
     });
   });