.NET: Add AIContextProvider message event and fix internal agent issue

[XiongHaoTrigger]

Motivation and Context

When a ChatClientAgent is used directly, messages injected by AIContextProvider are included in the agent session history and are preserved when the session is serialized.

However, when the same agent is wrapped with AgentWorkflowBuilder.BuildSequential(...).AsAIAgent(), the workflow session only persisted the original user input and the final workflow response. The request-side messages injected by the inner agent's AIContextProvider stayed inside the inner agent session and were not copied into the outer WorkflowSession.

This caused the workflow-as-agent session history and serialized state to lose important contextual messages.

Fixes #6344.

Description

This change adds an internal workflow event for AIContextProvider-injected request messages.

AIAgentHostExecutor now captures the inner agent session history before and after invoking the hosted agent. Any newly added request messages marked with AgentRequestMessageSourceType.AIContextProvider are emitted as an internal workflow event.

WorkflowSession handles this internal event by adding those messages to the workflow chat history. The messages are persisted into the outer workflow session, but they are not yielded as user-visible output and are not forwarded to downstream agents in the workflow.

New regression tests verify that:

• AIContextProvider-injected messages are persisted in workflow-as-agent chat history.
• The messages survive workflow session serialization.

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

Note

Copilot was unable to run its full agentic suite in this review.

Adds support for persisting AIContextProvider-injected request messages into workflow chat history (and session serialization) while preventing those injected messages from being forwarded to downstream agents.

Changes:

• Emit a workflow event for AIContextProvider-injected request messages detected during agent invocation.
• Persist injected request messages into WorkflowSession chat history when the new event is observed.
• Add unit tests covering serialization persistence and non-forwarding to downstream agents.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/AIContextProviderWorkflow.cs	Adds regression tests for persisting injected context into workflow history/serialization and preventing downstream leakage
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowSession.cs	Handles a new workflow event to persist AIContextProvider request messages into workflow chat history
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/AIAgentHostExecutor.cs	Detects injected request messages by diffing stored chat history and emits an event to the workflow
dotnet/src/Microsoft.Agents.AI.Workflows/AgentAIContextProviderMsgEvent.cs	Introduces an internal workflow event carrying injected request messages