.NET: Pass through external input request and handle response conversion for workflow as agent scenario

[peibekwe]

Description

This PR handles external input request for non-Function call contents the right way and also fixes issue with external input response conversion from checkpoints.

Fixes #3029

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

This PR improves the “workflow as agent” experience in the .NET workflows layer by preserving the original external-request content types (instead of always converting to FunctionCallContent) and enabling inbound external responses to be correctly converted back into ExternalResponse during workflow resume from checkpoints.

Changes:

• Preserve original FunctionCallContent / UserInputRequestContent when emitting RequestInfoEvent updates to callers.
• Track pending external requests in WorkflowSession.StateBag and convert inbound FunctionResultContent / UserInputResponseContent into ExternalResponse when resuming a run.
• Add unit tests covering preservation of request content types and basic request/response roundtrip acceptance.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowSession.cs	Tracks pending requests and converts inbound response contents into ExternalResponse on resume; preserves original request content type in streamed updates.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/WorkflowHostSmokeTests.cs	Adds tests to validate content-type preservation and roundtrip response processing behavior.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.