.NET: fix service-stored history provider resolution

[he-yufeng]

Summary

• skip the default chat history provider once a session is backed by a service conversation id
• keep per-service-call history persistence working when it uses its sentinel conversation id
• add a regression assertion that service-stored history does not populate the default in-memory provider

Fixes #6120.

To verify

• dotnet test --project tests\Microsoft.Agents.AI.UnitTests\Microsoft.Agents.AI.UnitTests.csproj -f net10.0 -v Minimal --filter-class Microsoft.Agents.AI.UnitTests.ChatClientAgent_ChatHistoryManagementTests
• dotnet test --project tests\Microsoft.Agents.AI.UnitTests\Microsoft.Agents.AI.UnitTests.csproj -f net472 -v Minimal --filter-class Microsoft.Agents.AI.UnitTests.ChatClientAgent_ChatHistoryManagementTests
• dotnet format agent-framework-dotnet.slnx --verify-no-changes --no-restore --include src\Microsoft.Agents.AI\ChatClient\ChatClientAgent.cs tests\Microsoft.Agents.AI.UnitTests\ChatClient\ChatClientAgent_ChatHistoryManagementTests.cs
• git diff --check