.NET: Honor cancellation for Foundry-hosted workflow responses

[Copilot]

Motivation & Context

Workflow cancellation can end the underlying .NET workflow stream normally, causing the hosting adapter to emit response.completed instead of allowing AgentServer to finalize the response as cancelled.

Description & Review Guide

• What are the major changes?
  • Recheck the linked cancellation token before accepting a successful terminal event.
  • Add in-flight cancellation coverage for regular and workflow-backed agents.
• What is the impact of these changes?
  • Explicit cancellation no longer produces a false successful completion.
  • Existing steering, consent, and shutdown handling remains unchanged.
• What do you want reviewers to focus on?
  • Cancellation classification around workflow stream exhaustion and terminal-event emission.

Related Issue

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.

• Fixes Foundry hosting: WorkflowAgent path drops cancellation_signal #7511

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