.NET: Add IChatMessageInjector for message injection during function loop

[westey (westey-m)]

Summary

Adds the ability to inject user messages during the function execution loop in PerServiceCallChatHistoryPersistingChatClient. This enables scenarios where tool code can enqueue follow-up messages that the model processes automatically — without requiring additional user turns.

Changes

New: IChatMessageInjector interface

• Experimental public interface discoverable via GetService<IChatMessageInjector>()
• Single method: EnqueueMessages(IEnumerable<ChatMessage> messages)

Updated: PerServiceCallChatHistoryPersistingChatClient

• Implements IChatMessageInjector with a thread-safe ConcurrentQueue<ChatMessage>
• DrainInjectedMessages prepends queued messages (never mutates the incoming list)
• Internal loop in both GetResponseAsync and GetStreamingResponseAsync: when no actionable FunctionCallContent remains but injected messages are pending, loops and calls the service again
• HasActionableFunctionCalls helper checks for non-InformationalOnly FCC

New: Unit tests

• 7 tests covering: GetService resolution, message inclusion, drain behavior, internal loop trigger, no-loop with actionable FCC, loop with InformationalOnly FCC

Updated: Agent_Step19_InFunctionLoopCheckpointing sample

• Replaced artificial InjectFollowUp tool with natural CheckTravelAdvisory tool
• Advisory detection injects a follow-up asking for alternative destinations
• 4th turn added to both non-streaming and streaming modes demonstrating the feature

[Copilot]

Pull request overview

Adds an experimental message-injection capability to the .NET ChatClientAgent stack so tool/code can enqueue additional user messages during a function-calling run, and the chat client can internally re-call the service to process them without requiring a new user turn.

Changes:

• Introduces experimental IChatMessageInjector and makes it discoverable via GetService<IChatMessageInjector>() when per-service-call persistence middleware is active.
• Updates PerServiceCallChatHistoryPersistingChatClient to enqueue/drain injected messages and to internally loop when injected messages are pending (both non-streaming and streaming).
• Adds unit tests and updates the Step19 sample to demonstrate tool-driven follow-up injection.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 8 comments.

File	Description
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/PerServiceCallChatHistoryPersistingChatClientTests.cs	Adds tests for injector discovery, queue draining, and internal looping behavior.
dotnet/src/Microsoft.Agents.AI/ChatClient/PerServiceCallChatHistoryPersistingChatClient.cs	Implements IChatMessageInjector, message queue draining, and internal looping in both response paths.
dotnet/src/Microsoft.Agents.AI/ChatClient/IChatMessageInjector.cs	Adds new experimental public interface for message injection.
dotnet/samples/02-agents/Agents/Agent_Step19_InFunctionLoopCheckpointing/Program.cs	Demonstrates tool-based injection using the ambient run context.

[github-actions]

Automated Code Review

Reviewers: 4 | Confidence: 88% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Design Approach

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Automated review by westey-m's agents