Adds integration test coverage for the GetOrchestrationHistoryAsync method as requested in #516.

Test Coverage

• Completed orchestration: Verifies history contains expected events (ExecutionStartedEvent, ExecutionCompletedEvent, EventRaisedEvent) with correct data
• Non-existent orchestration: Verifies ArgumentException is thrown with appropriate message

Test Host Enhancements

To support these tests, enhanced StreamInstanceHistory in the test sidecar:

• InMemoryOrchestrationService.TryGetOrchestrationHistory — retrieves history from instance store for completed orchestrations
• TaskHubGrpcServer.StreamInstanceHistory — now checks streaming cache first, falls back to instance store, throws NotFound when instance doesn't exist

[Fact]
public async Task GetOrchestrationHistoryAsync_CompletedOrchestration_ReturnsHistoryEvents()
{
    // ... setup orchestration and complete it ...
    
    IList<HistoryEvent> history = await server.Client.GetOrchestrationHistoryAsync(instanceId, cts.Token);
    
    history.Should().Contain(e => e is ExecutionStartedEvent);
    history.Should().Contain(e => e is ExecutionCompletedEvent);
}

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