.NET: add workflow agent tool call message filtering

[Oxygen56]

Motivation & Context

Fixes #6874.

When a workflow is hosted as an AIAgent, internal tool-call messages from nested agent execution can be surfaced in the hosted agent response. If that response is passed to another agent, the downstream provider can receive FunctionCallContent / FunctionResultContent for tool calls it did not issue and reject the request.

Description & Review Guide

• What are the major changes?
  • Adds a non-breaking AsAIAgent(filterToolCallMessages: true, ...) overload.
  • Carries the option into WorkflowHostAgent / WorkflowSession.
  • Filters FunctionCallContent and FunctionResultContent from streamed agent response updates, aggregated agent response messages, and workflow output chat messages before they are surfaced by the hosted workflow agent.
  • Keeps the default behavior unchanged.
• What is the impact of these changes?
  • Existing callers continue to see tool-call content unless they opt in.
  • Opted-in hosted workflow agents expose regular response content without internal tool-call IDs.
• What do you want reviewers to focus on?
  • Whether the option should apply to all hosted workflow chat outputs handled here, or only the AgentResponseUpdateEvent path.
  • The overload shape chosen to preserve the existing public signature.

Related Issue

Fixes #6874

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

Adds an opt-in switch to the .NET workflow-hosted agent surface to filter tool-call transcript content (function call/result contents) from surfaced messages, with unit coverage to validate default vs filtered behavior.

Changes:

• Introduces filterToolCallMessages option on Workflow.AsAIAgent(...) and threads it through WorkflowHostAgent → WorkflowSession.
• Implements filtering in WorkflowSession so FunctionCallContent / FunctionResultContent are removed from streamed updates and message-derived updates.
• Adds smoke tests covering default behavior (preserves tool call contents) and filtered behavior (removes tool call contents while preserving text).

Reviewed changes

Copilot reviewed 5 out of 5 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 test workflows and assertions validating tool-call content preservation vs filtering in both streaming and non-streaming runs.
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowSession.cs	Implements the core filtering logic and applies it when yielding AgentResponseUpdates from workflow events.
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowHostingExtensions.cs	Adds a new AsAIAgent overload exposing the filterToolCallMessages toggle.
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowHostAgent.cs	Propagates filterToolCallMessages into created/deserialized WorkflowSessions.
dotnet/src/Microsoft.Agents.AI.Workflows/Futures.cs	Updates XML doc cref to a specific AsAIAgent overload signature.