Add strongly-typed external events with DurableEventAttribute

[Copilot]

Enables compile-time type-safe external event handling through source-generated extension methods.

Changes

• DurableEventAttribute: New attribute for annotating event types (classes/records/structs)
• Source Generator: Extended to detect DurableEventAttribute and generate WaitFor{EventName}Async extension methods on TaskOrchestrationContext
• Tests: Added unit tests covering records, classes, and namespaced event types
• Sample: EventsSample project demonstrating approval and data processing workflows

Usage

Define an event:

[DurableEvent(nameof(ApprovalEvent))]
public sealed record ApprovalEvent(bool Approved, string? Approver);

Generated extension method:

public static Task<ApprovalEvent> WaitForApprovalEventAsync(
    this TaskOrchestrationContext context, 
    CancellationToken cancellationToken = default)

Use in orchestrator:

// Before: string literal and explicit generic type
ApprovalEvent approval = await context.WaitForExternalEvent<ApprovalEvent>("ApprovalEvent");

// After: strongly-typed method with IntelliSense
ApprovalEvent approval = await context.WaitForApprovalEventAsync();

Attribute supports custom event names: [DurableEvent("CustomName")]

Original prompt

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This section details on the original issue you should resolve

<issue_title>Strongly-typed events</issue_title>
<issue_description>It would be nice to have methods for external events generated the same way we get orchestrations and activities added to TaskOrchestrationContext.

Example: If I define below type and annotate it with the (proposed) DurableEventAttribute

[DurableEvent(nameof(ApprovalEvent))]
public sealed record ApprovalEvent(bool Approved, string? Approver);

The generator should generate the following extension method:

var approvalResult = await context.WaitForApprovalEvent(); // approvalResult is of type ApprovalEvent

</issue_description>

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

• Fixes Strongly-typed events #425

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