.NET: Expand Workflow Unit Test Coverage

[lokitoth]

Description

• Adds unit tests for ChatForwardingExecutor, FunctionExecutor
• Adds test for failure when trying to host non-ChatProtocol workflow as an AIAgent
• Remove dead code and suppress coverage for HandoffsWorkflowBuilder (obsolete name, all fuctionality covered by existing tests targeting HandoffWorkflowBuilder)
• Obsolete unused attributes
• Contributes to .NET: Improve unit test coverage for Microsoft.Agents.AI.Workflows #3342

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• 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

Expands .NET workflow unit test coverage (targeting the Workflows library) and removes/adjusts some legacy or dead code to help meet the required coverage threshold.

Changes:

• Added new unit tests for ChatForwardingExecutor and FunctionExecutor, plus a smoke test ensuring non-ChatProtocol workflows cannot be hosted as an AIAgent.
• Updated FunctionExecutor to correctly incorporate IO attributes ([SendsMessage] / [YieldsOutput]) for synchronous delegates.
• Removed unused public attributes / helper code and excluded an obsolete builder type from code coverage.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/WorkflowHostSmokeTests.cs	Adds a regression test that hosting a non-ChatProtocol workflow as an agent fails.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/FunctionExecutorTests.cs	New tests validating delegate invocation and protocol registration behavior for FunctionExecutor.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/ChatForwardingExecutorTests.cs	New tests validating forwarding behavior across supported message shapes and string-role configuration.
dotnet/src/Microsoft.Agents.AI.Workflows/StreamsMessageAttribute.cs	Removes an unused public attribute type.
dotnet/src/Microsoft.Agents.AI.Workflows/HandoffWorkflowBuilder.cs	Excludes obsolete HandoffsWorkflowBuilder from coverage due to 1:1 equivalence with the preferred type.
dotnet/src/Microsoft.Agents.AI.Workflows/FunctionExecutor.cs	Fixes sync-delegate constructors to capture IO attribute types for protocol declaration.
dotnet/src/Microsoft.Agents.AI.Workflows/ChatForwardingExecutor.cs	Uses configured StringMessageChatRole when converting string inputs to ChatMessage.
dotnet/src/Microsoft.Agents.AI.Workflows/Attributes/YieldsMessageAttribute.cs	Removes an unused public attribute type (legacy name vs YieldsOutputAttribute).
dotnet/src/Microsoft.Agents.AI.Workflows/AIAgentsAbstractionsExtensions.cs	Removes an unused internal ToChatMessage conversion helper.
dotnet/src/Microsoft.Agents.AI.Workflows.Generators/Analysis/SemanticAnalyzer.cs	Updates documentation comment to reflect .YieldsOutput protocol generation.