.NET: feat: add WorkflowCompletedEvent and WorkflowFailedEvent (#4063)

[max-montes]

.NET: Add WorkflowCompletedEvent and WorkflowFailedEvent to core Workflows library

Add two new public workflow event types to the core Workflows library:

• WorkflowCompletedEvent: emitted when a workflow completes execution, providing the natural counterpart to
  WorkflowStartedEvent. Consumers can now observe workflow completion through the event stream instead of inferring it from stream termination.
• WorkflowFailedEvent: supports string-based failure scenarios where no Exception object is available (e.g., errors from
  external orchestrators or deserialized error responses). Complements the existing WorkflowErrorEvent which requires an Exception.

WorkflowCompletedEvent is automatically emitted in both streaming (StreamingRunEventStream) and lockstep (
LockstepRunEventStream) execution modes before the event stream terminates.

RequestHaltEvent remains internal as it serves as infrastructure plumbing for stream termination signaling.

Closes #4063

Motivation and Context

Issue #4063 identified that when consuming workflow events via WatchStreamAsync(), there was no way to observe workflow
completion or string-based failures through the event stream. This change adds that observability, following the patterns
established by @kshyju's durable workflow events in PR #4020.

Description

New files:

• WorkflowCompletedEvent.cs — sealed class with optional object? result, extends WorkflowEvent
• WorkflowFailedEvent.cs — sealed class with string ErrorMessage, extends WorkflowEvent

Modified files:

• WorkflowEvent.cs — added [JsonDerivedType] attributes for both new types
• StreamingRunEventStream.cs — emit WorkflowCompletedEvent before yield break on both RequestHaltEvent and InternalHaltSignal(Idle/Ended) termination paths
• LockstepRunEventStream.cs — emit WorkflowCompletedEvent before yield break on halt and after natural do-while completion

Tests: 6 new tests covering construction, both execution modes (OffThread + Lockstep), and executor-emitted failure events.
All 252 tests pass.

Note: Adding ExecutorId to RequestHaltEvent (item 3 of the issue) would require changes to InProcessRunnerContext internals —
noted as a potential follow-up.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines (https://github.com/microsoft/agent-framework/blob/main/CONTRIBUTING.md)
• All unit tests pass, and I have added new tests where possible
• [] Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

Adds explicit terminal workflow events to the .NET Workflows core library so consumers can observe workflow completion and string-based failures via the workflow event stream, aligning with existing workflow observability patterns.

Changes:

• Introduces new public workflow event types: WorkflowCompletedEvent and WorkflowFailedEvent.
• Updates workflow event polymorphic serialization metadata to include the new event types.
• Emits WorkflowCompletedEvent as a terminal event in both streaming (off-thread) and lockstep event-stream implementations, with new unit tests validating behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/WorkflowCompletedEventTests.cs	Adds unit tests for the new events and verifies completion event is last in both execution modes.
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowFailedEvent.cs	Adds a string-based failure event type for scenarios without an Exception.
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowEvent.cs	Registers the new event types for polymorphic JSON serialization.
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowCompletedEvent.cs	Adds a workflow completion event type carrying an optional result payload.
dotnet/src/Microsoft.Agents.AI.Workflows/Execution/StreamingRunEventStream.cs	Emits WorkflowCompletedEvent before terminating the streaming event stream.
dotnet/src/Microsoft.Agents.AI.Workflows/Execution/LockstepRunEventStream.cs	Emits WorkflowCompletedEvent before terminating the lockstep event stream.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.