.NET: Persist messages during function call loop

[westey-m]

Motivation and Context

Allow persisting messages during the function call loop when using ChatHistoryProvider. This allows us to match the behavior of service stored chat history, e.g. responses with store=true, which persists chat history with each call.

#2889

Description

• Introduce a ChatClient decorator that allows persisting messages to a ChatHistoryProvider during the function call loop.
• Introduce a setting on ChatClientAgentOptions that allows switching on this behavior.

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

Adds an opt-in mechanism for persisting chat history (and notifying context providers) after each individual service call within a ChatClientAgent run, including during tool/function-invocation loops—improving recoverability and observability when runs are interrupted.

Changes:

• Introduces ChatHistoryPersistingChatClient decorator and wires it into the default chat client middleware pipeline when enabled.
• Adds ChatClientAgentOptions.PersistChatHistoryAfterEachServiceCall and adjusts ChatClientAgent to skip end-of-run persistence when per-call persistence is enabled.
• Adds unit tests and a new Step 19 sample demonstrating in-function-loop checkpointing.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatHistoryPersistingChatClient.cs	New delegating chat client that notifies providers per service call and deduplicates already-persisted messages.
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientExtensions.cs	Injects the new decorator into the default middleware pipeline when the option is enabled.
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentOptions.cs	Adds the new opt-in option (marked experimental) and includes it in Clone().
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Refactors provider notification to support both end-of-run and per-service-call flows.
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentSession.cs	Adds new using directives (currently introduces build-breaking unused usings).
dotnet/src/Microsoft.Agents.AI/Compaction/SummarizationCompactionStrategy.cs	Adds a file-level suppression for CA1873.
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatHistoryPersistingChatClientTests.cs	New unit tests validating per-call persistence, deduping, and decorator injection behavior.
dotnet/samples/02-agents/Agents/README.md	Adds sample entry for Step 19.
dotnet/samples/02-agents/Agents/Agent_Step19_InFunctionLoopCheckpointing/*	New sample (README + Program + csproj) demonstrating the new option.
dotnet/agent-framework-dotnet.slnx	Adds the new sample project to the solution.