.NET: Added support for polymorphic type as workflow output

[peibekwe]

Description

Added support for polymorphic type as workflow output and enable executors to return instance of derived class which inherits from the base class.

Fixes #4134

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

Adds support for workflow outputs to be validated polymorphically so executors can return derived instances when their declared workflow output type is a base class (fixing issue #4134).

Changes:

• Update executor output validation to accept derived types for declared yield/output types.
• Add unit tests covering derived, multi-level derived, exact-type, and unrelated-type output behaviors.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
dotnet/src/Microsoft.Agents.AI.Workflows/Executor.cs	Adjusts CanOutput logic to validate outputs polymorphically (base-class inheritance).
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/PolymorphicOutputTests.cs	Adds regression coverage for derived outputs and ensures unrelated outputs still fail.