Add cancellation support for activities, sub-orchestrators, and retry handlers

[Copilot]

The TaskOptions class accepted a CancellationToken parameter but it was not wired up to actually cancel tasks.

Changes

Added CancellationToken property to TaskOptions

• Includes XML documentation with usage examples for activity cancellation and retry handler integration
• Property flows through SubOrchestrationOptions via base constructor

Implemented cancellation in TaskOrchestrationContextWrapper

• Pre-scheduling check: throws TaskCanceledException if token is already cancelled
• Post-scheduling cancellation: WaitForTaskWithCancellation uses Task.WhenAny pattern to race task completion against cancellation
• Retry handler integration: passes CancellationToken through RetryContext for imperative retry control
• Performance optimized: skips cancellation setup for already-completed tasks

Added integration tests

• Activity cancellation before scheduling
• Sub-orchestrator cancellation before scheduling
• Retry handler receives and respects cancellation token
• Retry handler stops retrying on cancellation

Usage

// Cancel activity after timeout
using CancellationTokenSource cts = new(TimeSpan.FromMinutes(5));
TaskOptions options = new() { CancellationToken = cts.Token };

try
{
    var result = await context.CallActivityAsync<string>("MyActivity", input, options);
}
catch (TaskCanceledException)
{
    // Handle cancellation
}

// Retry handler with cancellation
TaskOptions retryOptions = new(TaskOptions.FromRetryHandler(retryContext =>
{
    if (retryContext.CancellationToken.IsCancellationRequested)
        return false; // Stop retrying
    return retryContext.LastAttemptNumber < 3;
}))
{
    CancellationToken = cts.Token
};

Notes

• Cancellation is cooperative - does not terminate running activities/sub-orchestrators
• Implementation is replay-safe and deterministic per orchestrator constraints
• Parent orchestrator cancellation does not cascade to sub-orchestrations

Original prompt

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This section details on the original issue you should resolve

<issue_title>Activity and sub-orchestrator cancellation support</issue_title>
<issue_description>The TaskOptions class provided when scheduling activities and sub-orchestration calls supports providing a CancellationToken. However, it's not yet wired up to actually cancel the task. This issue tracks adding support for cancellation.

• Implement the feature for activity calls, sub-orchestrator calls, and retry handlers.
• Add detailed <remarks> documentation with examples for how cancellation can be used.
• Add tests that cover all the scenarios supported by task cancellation</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Activity and sub-orchestrator cancellation support #7

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