.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

[he-yufeng]

Rebased on current upstream/main and reran the scoped ChatClientAgent validation locally.

Validated:

dotnet build .\tests\Microsoft.Agents.AI.UnitTests\Microsoft.Agents.AI.UnitTests.csproj -f net10.0 -c Debug --tl:off
dotnet test --project .\tests\Microsoft.Agents.AI.UnitTests\Microsoft.Agents.AI.UnitTests.csproj -f net10.0 -c Debug --no-build -v Normal --report-xunit-trx --ignore-exit-code 8 -- --filter-class Microsoft.Agents.AI.UnitTests.ChatClient.ChatClientAgent_ChatHistoryManagementTests
git diff --check upstream/main...HEAD

All passed after the rebase.

[he-yufeng]

Correction: my first test command used the wrong class namespace and selected 0 tests. I reran with the actual class name.

Validated:

dotnet test --project .\tests\Microsoft.Agents.AI.UnitTests\Microsoft.Agents.AI.UnitTests.csproj -f net10.0 -c Debug --no-build -v Normal --report-xunit-trx --ignore-exit-code 8 -- --filter-class Microsoft.Agents.AI.UnitTests.ChatClientAgent_ChatHistoryManagementTests
git diff --check upstream/main...HEAD

Result: 19/19 tests passed.

[he-yufeng]

Rebased this PR onto the latest upstream main and re-ran the focused validation.

Validation:

• git diff --check upstream/main..HEAD
• dotnet build .\dotnet\tests\Microsoft.Agents.AI.UnitTests\Microsoft.Agents.AI.UnitTests.csproj -f net10.0 -c Debug --tl:off
• dotnet .\dotnet\tests\Microsoft.Agents.AI.UnitTests\bin\Debug\net10.0\Microsoft.Agents.AI.UnitTests.dll --filter-class Microsoft.Agents.AI.UnitTests.ChatClientAgent_ChatHistoryManagementTests --no-progress (19 passed)

[he-yufeng]

Rebased this PR onto the latest upstream main and resolved the ChatClientAgent.cs conflict by preserving the new AG-UI provider exception while keeping this PR's service-stored-history behavior for non-AG-UI providers. Validation after the rebase: git diff --check, dotnet build .\dotnet\tests\Microsoft.Agents.AI.UnitTests\Microsoft.Agents.AI.UnitTests.csproj -f net10.0 -c Debug --tl:off, and dotnet .\dotnet\tests\Microsoft.Agents.AI.UnitTests\bin\Debug\net10.0\Microsoft.Agents.AI.UnitTests.dll --filter-class Microsoft.Agents.AI.UnitTests.ChatClientAgent_ChatHistoryManagementTests --no-progress (19 passed).

[he-yufeng]

Rebased this branch onto current main and pushed the updated head 75113466.

Validation run locally from dotnet/:

• dotnet build .\src\Microsoft.Agents.AI\Microsoft.Agents.AI.csproj -f net10.0 --tl:off -> passed, 0 warnings/errors
• dotnet run --project .\tests\Microsoft.Agents.AI.UnitTests\Microsoft.Agents.AI.UnitTests.csproj -f net10.0 -> passed, 1647 tests
• git diff --check upstream/main..HEAD -> passed

I used dotnet run --project for the xUnit v3/MTP test project because dotnet test is not usable with this checkout's current runner configuration on my Windows machine.

[he-yufeng]

Rebased this branch onto current main and pushed the updated head $head.

Validation run locally from dotnet/:

• dotnet build .\src\Microsoft.Agents.AI\Microsoft.Agents.AI.csproj -f net10.0 --tl:off -> passed, 0 warnings/errors
• dotnet run --project .\tests\Microsoft.Agents.AI.UnitTests\Microsoft.Agents.AI.UnitTests.csproj -f net10.0 -> passed, 1647 tests
• git diff --check upstream/main..HEAD -> passed