.NET: Features/3768-devui-aspire-integration

[tommasodotNET]

Motivation and Context

Adds a new Aspire hosting library that provides a DevUI resource for testing and debugging AI agents built with Microsoft Agent Framework.

Closes #3768

Description

This PR introduces Aspire.Hosting.AgentFramework — an extension library that enables Aspire AppHost projects to spin up a unified DevUI for interacting with multiple agent services during development.

Key Features

• AddDevUI() extension method — Registers a DevUI resource in the Aspire AppHost
• WithAgentService() fluent API — Connects agent services to the DevUI with optional metadata
• In-process aggregator — A lightweight Kestrel server running inside the AppHost that:
  • Serves the DevUI frontend from embedded resources (no external container required)
  • Aggregates /v1/entities listings across all configured backends
  • Routes requests to the correct backend based on entity ID prefix
  • Streams SSE responses for real-time agent output

Usage

var writerAgent = builder.AddProject<Projects.WriterAgent>("writer-agent");
var editorAgent = builder.AddProject<Projects.EditorAgent>("editor-agent");

var devui = builder.AddDevUI("devui")
    .WithAgentService(writerAgent)
    .WithAgentService(editorAgent)
    .WaitFor(writerAgent)
    .WaitFor(editorAgent);

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.

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