Fix InvalidCastException for history event type mismatch

[cgillum]

While investigating microsoft/durabletask-mssql#198, I noticed that some of the new distributed tracing code casts history events based on certain assumptions, which aren't always true in production. The issue can result in a poison message with the following error:

System.InvalidCastException: Unable to cast object of type 'DurableTask.Core.History.TaskCompletedEvent' to type 'DurableTask.Core.History.TaskScheduledEvent'.
at DurableTask.Core.TaskOrchestrationDispatcher.ReconcileMessagesWithState(TaskOrchestrationWorkItem workItem, String dispatcher, ErrorPropagationMode errorPropagationMode, LogHelper logHelper) in //src/DurableTask.Core/TaskOrchestrationDispatcher.cs:line 860
at DurableTask.Core.TaskOrchestrationDispatcher.OnProcessWorkItemAsync(TaskOrchestrationWorkItem workItem) in //src/DurableTask.Core/TaskOrchestrationDispatcher.cs:line 345
at DurableTask.Core.TaskOrchestrationDispatcher.OnProcessWorkItemAsync(TaskOrchestrationWorkItem workItem)
at DurableTask.Core.TaskOrchestrationDispatcher.OnProcessWorkItemSessionAsync(TaskOrchestrationWorkItem workItem) in //src/DurableTask.Core/TaskOrchestrationDispatcher.cs:line 211
at DurableTask.Core.WorkItemDispatcher`1.ProcessWorkItemAsync(WorkItemDispatcherContext context, Object workItemObj) in //src/DurableTask.Core/WorkItemDispatcher.cs:line 373

This PR replaces the cast operation with a type-safe check, removing the possibility of InvalidCastException.

Note that the root cause of the bug observed by the user is probably specific to the MSSQL backend: it might not be deserializing history events correctly, resulting in unexpected Event ID matches. Regardless, I felt it made sense to make a fix here for defense in depth.

[bachuv]

Thanks for making this change!