Fix flaky DiagnosticTests on net10.0 by waiting for specific activities

[Copilot]

DiagnosticTests.Session_TracksActivities fails intermittently on net10.0 due to a race condition. Server messages are processed via fire-and-forget tasks, and the WaitForAsync conditions used activity count thresholds (>= 4) that could be satisfied by other server activities before the expected tools/call activity was exported.

• Changed all three WaitForAsync conditions to wait for the specific activities the assertions require, rather than a server activity count

// Before: count-based, racy
await WaitForAsync(() => activities.Count(a => a.Kind == ActivityKind.Server) >= 4);

// After: waits for the exact activity needed
await WaitForAsync(() => activities.Any(a =>
    a.DisplayName == "tools/call DoubleValue" && a.Kind == ActivityKind.Server));

This failure also reproduces on main — it is not caused by #1320.

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