.NET: [BREAKING] Add ability to mark the source of Agent request messages and use that for filtering

[westey-m]

Motivation and Context

This is addressing feedback that messages should be annotated with their source, rather than split by type when passed to the AIContextProvider and ChatHistoryProvider.

#2542

Description

• Add AdditionalProperties value to ChatMessage to annotate where it came from for certain cases
• Treat all chat history, ai context provider and user input as request messages, and filter using the new property.

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

This PR introduces explicit tracking of the source of agent request messages (external input, AI context provider, chat history), and updates the agent pipeline and providers to use this metadata for filtering and behavior.

Changes:

• Add AgentRequestMessageSource and ChatMessageExtensions.GetAgentRequestMessageSource to tag and retrieve the logical source of ChatMessage instances via AdditionalProperties.
• Refactor AIContextProvider and ChatHistoryProvider to use public wrapper methods (InvokingAsync/InvokedAsync) delegating to new protected InvokingCoreAsync / InvokedCoreAsync, and update all implementations (Mem0, Cosmos, in-memory, workflow, samples) and tests accordingly.
• Update ChatClientAgent and various providers (ChatHistoryMemoryProvider, TextSearchProvider, Mem0, ChatHistory filters) so they treat all messages uniformly as request messages, using the new source metadata to include or exclude messages from memory, search, and history operations.

Reviewed changes

Copilot reviewed 33 out of 33 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentRequestMessageSource.cs	Adds a new sealed type and key for annotating ChatMessage source via AdditionalProperties.
dotnet/src/Microsoft.Agents.AI.Abstractions/ChatMessageExtensions.cs	Adds an extension method to read the AgentRequestMessageSource from a ChatMessage, defaulting to External.
dotnet/src/Microsoft.Agents.AI.Abstractions/AIContextProvider.cs	Introduces InvokingCoreAsync/InvokedCoreAsync and a wrapper that tags AI-context messages with AIContextProvider source.
dotnet/src/Microsoft.Agents.AI.Abstractions/ChatHistoryProvider.cs	Introduces InvokingCoreAsync/InvokedCoreAsync and a wrapper that tags returned history messages with ChatHistory source, and simplifies InvokedContext.
dotnet/src/Microsoft.Agents.AI.Abstractions/ChatHistoryProviderExtensions.cs	Updates filters to work on the unified request-message list using AgentRequestMessageSource instead of separate AI-context collections.
dotnet/src/Microsoft.Agents.AI.Abstractions/ChatHistoryProviderMessageFilter.cs	Adjusts to new InvokingCoreAsync/InvokedCoreAsync pattern while preserving filtering behavior.
dotnet/src/Microsoft.Agents.AI.Abstractions/InMemoryChatHistoryProvider.cs	Updates core methods and switches to aggregating only request+response messages, relying on message-source metadata.
dotnet/src/Microsoft.Agents.AI/TextSearchProvider.cs	Uses message-source metadata to include only external-request text in search input and recent-message tracking.
dotnet/src/Microsoft.Agents.AI/Memory/ChatHistoryMemoryProvider.cs	Filters to external request messages when querying and storing vector-memory items.
dotnet/src/Microsoft.Agents.AI.Mem0/Mem0Provider.cs	Filters to external request messages for memory query/persist operations.
dotnet/src/Microsoft.Agents.AI.CosmosNoSql/CosmosChatHistoryProvider.cs	Migrates to new core methods and stops treating AI-context messages as a separate collection.
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Updates the run pipeline to use unified input-message lists, adjusts notifications to context/history providers, and removes explicit AI-context/history message collections in InvokedContext.
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowChatHistoryProvider.cs	Adapts workflow chat history to new core methods and unified message model.
dotnet/samples/... (multiple)	Updates custom AIContextProvider and ChatHistoryProvider implementations in samples to override the new *CoreAsync methods and to work with the unified request-message model.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/*.cs	Adds tests for AgentRequestMessageSource and ChatMessageExtensions, and updates existing tests to align with the new InvokedContext shape and message-source semantics.
dotnet/tests/Microsoft.Agents.AI.UnitTests/* and *.Mem0*, *.CosmosNoSql*	Adjusts tests to the new provider APIs and confirms that history, AI context, and memory providers behave correctly with the new source-tagging and filtering logic.

[rogerbarreto]

This conditional deserves a Method or Comment for clarity 😊