Remove dead continuedAsNew code after do-while loop

[cgillum]

Summary

The do...while (continuedAsNew) loop in OnProcessWorkItemAsync only exits when continuedAsNew is false. This means all references to continuedAsNew and continuedAsNewMessage after the loop are dead code:

• The ternary checks continuedAsNew ? null : messagesToSend always evaluate to messagesToSend (same for timerMessages).
• continuedAsNewMessage is always null after the loop (it's only assigned when continuedAsNew is true, which causes another loop iteration).
• continuedAsNew in the return statement is always false.

This code became dead ~7 years ago in #251, so it shouldn't be reflective of any recent change/regression.

Changes

• Narrowed the scope of continuedAsNew to just around the do-while loop
• Moved continuedAsNewMessage declaration inside the loop body
• Simplified the CompleteTaskOrchestrationWorkItemAsync call arguments
• Removed continuedAsNew from the return expression

Testing

• Build passes
• All 176 DurableTask.Core.Tests pass (82 net48 + 94 net6.0)

[Copilot]

Pull request overview

This PR removes dead code related to continuedAsNew orchestration handling after a do-while loop in TaskOrchestrationDispatcher.cs. The do-while loop only exits when continuedAsNew is false, making all references to continuedAsNew and continuedAsNewMessage after the loop dead code that can be safely removed.

Changes:

• Narrowed the scope of continuedAsNew and continuedAsNewMessage variables to prevent their use outside the loop where they would always have constant values
• Simplified the CompleteTaskOrchestrationWorkItemAsync method call by removing redundant null checks and passing concrete values instead of conditional expressions
• Removed the redundant continuedAsNew condition from the return statement

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