Respect unavailable AI chat client providers

[Copilot]

IChatClientProvider.IsAvailable documented that providers can be registered but unusable, but GetChatClientAsync() ignored it and still invoked client creation. Unavailable providers could therefore throw instead of behaving like no usable provider exists.

• Availability gate
  • GetChatClientAsync() now returns null when the registered provider reports IsAvailable == false.
  • Available providers continue to create clients as before.

return provider is null || !provider.IsAvailable
    ? null
    : await provider.CreateChatClientAsync(cancellationToken).ConfigureAwait(false);

• Regression coverage
  • Added unit coverage verifying an unavailable provider returns null and does not call CreateChatClientAsync().

[Copilot]

Pull request overview

This PR updates the Microsoft.Testing.Platform AI integration to honor IChatClientProvider.IsAvailable, ensuring that registered-but-unusable providers behave the same as “no provider registered” (i.e., GetChatClientAsync() returns null rather than attempting client creation and potentially throwing).

Changes:

• Gate GetChatClientAsync() on IChatClientProvider.IsAvailable to return null when unavailable.
• Add a unit test covering the “unavailable provider” scenario.
• Add a unit test project reference to Microsoft.Testing.Platform.AI to enable compiling the new AI-related unit test.

Show a summary per file

File	Description
test/UnitTests/Microsoft.Testing.Platform.UnitTests/Microsoft.Testing.Platform.UnitTests.csproj	Adds a project reference to the AI project (with netstandard targeting for non-.NETCoreApp TFMs) so unit tests can compile against AI types.
test/UnitTests/Microsoft.Testing.Platform.UnitTests/AI/ChatClientProviderExtensionsTests.cs	Introduces a regression test ensuring an unavailable provider results in null.
src/Platform/Microsoft.Testing.Platform.AI/ChatClientProviderExtensions.cs	Updates GetChatClientAsync() to return null when a provider exists but reports IsAvailable == false.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 2

[Evangelink]

@copilot address review comments

[Copilot]

@copilot address review comments

Addressed in e854d72: updated the XML docs for the nullable result and strengthened the regression test to assert the provider is resolved via IsAvailable and CreateChatClientAsync is not invoked.