test(dashboard-agent): fix flaky phase duration comparison

## Summary

Stop the duration-format equivalence test from comparing independently
generated observation timestamps. All other timeline fields remain
covered by the equality assertion.

Verified: all 49 tests in tool-curation.test.ts pass.
Mono-RevId: c679c63a9555eaf894d86aa6522026ec3584f3a9

Author: carderne

internal-packages/dashboard-agent/src/tool-curation.test.ts

@@ -823,7 +823,11 @@ describe("derivePhases", () => {
       },
     };
 
-    expect(derivePhases(agentTrace, run, "agent")).toEqual(derivePhases(trace, run, "legacy"));
+    const agentTimeline = derivePhases(agentTrace, run, "agent")!;
+    const legacyTimeline = derivePhases(trace, run, "legacy")!;
+
+    // Each call stamps its own observation time, independent of the duration format.
+    expect(agentTimeline).toEqual({ ...legacyTimeline, asOf: agentTimeline.asOf });
   });
 
   it("flags truncation when the phase cap itself dropped spans", () => {