.NET: Add IHostedAgentBuilder extensions for mapping A2A and OpenAI Responses

[DeagleGross]

Motivation and Context

There are already extensions to map OpenAI ChatCompletions via IHostedAgentBuilder like:

var pirateAgentBuilder = builder.AddAIAgent(...);
...
app.MapOpenAIChatCompletions(pirateAgentBuilder);

But there are none for A2A or OpenAI Responses. Adding those in this PR.

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 adds support for IHostedAgentBuilder overloads to mapping methods for both OpenAI Responses and A2A endpoints, enabling developers to use builder instances instead of agent names or instances when configuring routes. The changes also include a namespace correction and project reference updates.

• Added MapOpenAIResponses overloads accepting IHostedAgentBuilder
• Added MapA2A overloads accepting IHostedAgentBuilder
• Comprehensive test coverage for all new extension method overloads

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
EndpointRouteBuilderExtensionsTests.cs	Added 8 new tests for MapOpenAIResponses with IHostedAgentBuilder parameter
EndpointRouteA2ABuilderExtensionsTests.cs	New test file with comprehensive coverage (26 tests) for MapA2A extension methods
MessageConverterTests.cs	Fixed namespace from Microsoft.Agents.AI.Hosting.A2A.Tests.Converters to Microsoft.Agents.AI.Hosting.A2A.UnitTests.Converters
EndpointRouteBuilderExtensions.Responses.cs	Added two new MapOpenAIResponses overloads that accept IHostedAgentBuilder
EndpointRouteBuilderExtensions.cs	Added four new MapA2A overloads that accept IHostedAgentBuilder and added null argument validation
Microsoft.Agents.AI.Hosting.A2A.UnitTests.csproj	Added project reference to AspNetCore project
Microsoft.Agents.AI.Hosting.A2A.csproj	Updated InternalsVisibleTo from .Tests to .UnitTests
AgentWebChat.AgentHost.csproj	Fixed indentation (tabs to spaces)
agent-framework-dotnet.slnx	Updated test project path reference

[javiercn]

LGTM