.NET: assign AgentCard's URL to mapped-endpoint if not defined explicitly

[DeagleGross]

Motivation and Context

When users call app.MapA2A(agent, "/my/path", agentCard) supplying agent card, they can set a different agentCard's url. But in most cases that is not the intention, so we should return the default path per endpoint mapped by user as in the A2A protocol.

On example:

1. user maps agent via A2A on path "/my/path": app.MapA2A(agent, "/my/path", agentCard).
2. A2A SDK automatically adds the endpoint "/my/path/v1/card" to the API surface, where agent card will be returned
3. User expects to see the same url in agentCard by default (<baseaddress>/my/path/v1/card)

I've adjusted the helper code, where if we see that agentCard url is not filled in, then we set it to the user-configured endpoint.

Fixes #1765

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 pull request adds conformance testing for OpenAI tools/function calling serialization and enhances A2A agent card URL handling with automatic path completion. Additionally, it refactors test infrastructure to improve code reuse.

• Adds comprehensive conformance tests for tools serialization/deserialization in the OpenAI Chat Completions API
• Fixes JSON serialization for the Tool type by using JsonPolymorphic attributes instead of explicit JsonPropertyName on type discriminator
• Enhances A2A MapA2A extension to automatically append /v1/card suffix to agent card URLs when not present
• Refactors DummyChatClient to a shared Internal namespace for reuse across A2A unit tests
• Adds integration test to verify A2A card endpoint returns properly formatted agent cards with URLs

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/OpenAIChatCompletionsConformanceTests.cs	Adds new test method ToolsSerializationDeserializationAsync to verify tools are properly serialized in requests and deserialized in responses
dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests.csproj	Removes unnecessary Content Remove entries for trace files
dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/ConformanceTraces/ChatCompletions/tools/response.json	Adds expected response JSON for tools conformance test
dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/ConformanceTraces/ChatCompletions/tools/request.json	Adds request JSON with tools definitions for conformance test
dotnet/tests/Microsoft.Agents.AI.Hosting.A2A.UnitTests/Microsoft.Agents.AI.Hosting.A2A.UnitTests.csproj	Changes SDK to Web and adds Microsoft.AspNetCore.TestHost package reference
dotnet/tests/Microsoft.Agents.AI.Hosting.A2A.UnitTests/Internal/DummyChatClient.cs	Extracts DummyChatClient to shared Internal namespace for reuse
dotnet/tests/Microsoft.Agents.AI.Hosting.A2A.UnitTests/EndpointRouteA2ABuilderExtensionsTests.cs	Refactors to use shared DummyChatClient from Internal namespace
dotnet/tests/Microsoft.Agents.AI.Hosting.A2A.UnitTests/A2AIntegrationTests.cs	Adds integration test verifying A2A card endpoint behavior with URL population
dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/ChatCompletions/Models/Tool.cs	Changes Type property from JsonPropertyName to JsonIgnore, relying on JsonPolymorphic for type discrimination
dotnet/src/Microsoft.Agents.AI.Hosting.A2A/AIAgentExtensions.cs	Enhances MapA2A to automatically append /v1/card to agent card URLs when missing
dotnet/samples/AgentWebChat/AgentWebChat.AgentHost/Program.cs	Updates MapA2A calls to use agent builders instead of agent names for better type safety