.NET: fix: preserve AG-UI session history

[he-yufeng]

Summary

• keep ChatClientAgent's local history provider active for AG-UI thread ids
• avoid treating AG-UI ConversationId as service-managed model history
• add a two-turn AG-UI regression test for sessions after the thread id is set

Fixes #5898

Verified

• dotnet test --project tests\Microsoft.Agents.AI.AGUI.UnitTests\Microsoft.Agents.AI.AGUI.UnitTests.csproj -f net10.0 --filter-method '*RunStreamingAsync_WithSession_SendsFullHistoryAfterThreadIdIsSetAsync'
• dotnet test --project tests\Microsoft.Agents.AI.AGUI.UnitTests\Microsoft.Agents.AI.AGUI.UnitTests.csproj -f net10.0 --filter-class Microsoft.Agents.AI.AGUI.UnitTests.AGUIAgentTests
• dotnet test --project tests\Microsoft.Agents.AI.UnitTests\Microsoft.Agents.AI.UnitTests.csproj -f net10.0 --filter-class Microsoft.Agents.AI.UnitTests.ChatClientAgent_ChatHistoryManagementTests
• dotnet test --project tests\Microsoft.Agents.AI.AGUI.UnitTests\Microsoft.Agents.AI.AGUI.UnitTests.csproj -f net472 --filter-class Microsoft.Agents.AI.AGUI.UnitTests.AGUIAgentTests
• dotnet test --project tests\Microsoft.Agents.AI.UnitTests\Microsoft.Agents.AI.UnitTests.csproj -f net472 --filter-class Microsoft.Agents.AI.UnitTests.ChatClientAgent_ChatHistoryManagementTests
• dotnet test --project tests\Microsoft.Agents.AI.AGUI.UnitTests\Microsoft.Agents.AI.AGUI.UnitTests.csproj -f net10.0
• dotnet test --project tests\Microsoft.Agents.AI.AGUI.UnitTests\Microsoft.Agents.AI.AGUI.UnitTests.csproj -f net472

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[rogerbarreto]

Please use a static private method instead. The Xml comment doesn't align with the member logic.

Suggested change

	// AG-UI uses ConversationId as a thread id, not as a signal that the service stores model history.
	private bool ConversationIdIndicatesServiceManagedHistory =>
	!string.Equals(this._agentMetadata.ProviderName, AGUIProviderName, StringComparison.Ordinal);
	// ???
	private static bool IsAGUIProviderName(string providerName) =>
	string.Equals(providerName, AGUIProviderName, StringComparison.Ordinal);

[he-yufeng]

Addressed in ef6343528: replaced the instance property with a private static IsAGUIProviderName(...) helper and removed the misleading comment. I used string? for the parameter because ProviderName is nullable under the project settings; the comparison still behaves the same for non-AG-UI providers.

Validation passed locally: AGUI unit-test project build, 3 focused AG-UI history tests, full AGUI unit suite on net10.0 and net472, dotnet format --verify-no-changes, and git diff --check.