.NET: [BREAKING] Update message source code to match python.

[westey-m]

Motivation and Context

Updating the way in which source information are added for messages to match python.

Description

• Update message source code to match python.

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

Aligns the .NET message “source” stamping schema with the Python implementation by moving from separate source keys to a single attribution object (source type + source id) stored in ChatMessage.AdditionalProperties.

Changes:

• Introduces AgentRequestMessageSourceAttribution and updates providers to stamp messages with a single "attribution" entry (type + id).
• Renames message source extension API to GetAgentRequestMessageSourceType() and adds GetAgentRequestMessageSourceId().
• Updates all affected unit tests and message-filtering logic to use the new attribution model.

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentRequestMessageSourceAttribution.cs	Adds new attribution struct + key used for message stamping.
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentRequestMessageSourceType.cs	Converts source type to a struct and updates equality/default behavior.
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentRequestMessageSource.cs	Removes legacy source key constant.
dotnet/src/Microsoft.Agents.AI.Abstractions/ChatMessageExtensions.cs	Renames source getter and adds source-id getter based on attribution.
dotnet/src/Microsoft.Agents.AI.Abstractions/ChatHistoryProvider.cs	Updates chat-history stamping to write/read attribution object.
dotnet/src/Microsoft.Agents.AI.Abstractions/ChatHistoryProviderExtensions.cs	Updates filters to use GetAgentRequestMessageSourceType().
dotnet/src/Microsoft.Agents.AI.Abstractions/AIContextProvider.cs	Updates AI-context stamping to write/read attribution object.
dotnet/src/Microsoft.Agents.AI/TextSearchProvider.cs	Updates filtering to use new source-type accessor.
dotnet/src/Microsoft.Agents.AI/Memory/ChatHistoryMemoryProvider.cs	Updates filtering to use new source-type accessor.
dotnet/src/Microsoft.Agents.AI.Mem0/Mem0Provider.cs	Updates filtering to use new source-type accessor.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentRequestMessageSourceAttributionTests.cs	Adds coverage for attribution struct semantics.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentRequestMessageSourceTypeTests.cs	Updates tests for struct/default semantics.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/ChatMessageExtensionsTests.cs	Updates tests and adds coverage for source-id accessor.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AIContextProviderTests.cs	Updates stamping assertions to validate attribution.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/ChatHistoryProviderTests.cs	Updates stamping assertions to validate attribution.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/InMemoryChatHistoryProviderTests.cs	Updates request message setup to use attribution.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/ChatHistoryProviderMessageFilterTests.cs	Updates filter logic and message setup to use new accessor/attribution.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/ChatHistoryProviderExtensionsTests.cs	Updates message setup to use attribution.
dotnet/tests/Microsoft.Agents.AI.CosmosNoSql.UnitTests/CosmosChatHistoryProviderTests.cs	Updates message setup to use attribution.

Comments suppressed due to low confidence (1)

dotnet/src/Microsoft.Agents.AI.Abstractions/AgentRequestMessageSourceType.cs:32

• Changing AgentRequestMessageSourceType from a sealed class to a readonly struct (and removing AdditionalPropertiesKey) is a binary/source breaking change and also changes null/default semantics for consumers. If this is intentional, the PR/package should be treated as breaking; otherwise consider reintroducing compatibility shims (e.g., keep the old type/constant with [Obsolete], or provide a type-forwarding wrapper) so existing integrations continue to compile/run.

public readonly struct AgentRequestMessageSourceType : IEquatable<AgentRequestMessageSourceType>
{
    /// <summary>
    /// Initializes a new instance of the <see cref="AgentRequestMessageSourceType"/> struct.
    /// </summary>
    /// <param name="value">The string value representing the source of the agent request message.</param>
    public AgentRequestMessageSourceType(string value) => this.Value = Throw.IfNullOrWhitespace(value);

    /// <summary>
    /// Get the string value representing the source of the agent request message.
    /// </summary>
    public string Value { get { return field ?? External.Value; } }

    /// <summary>
    /// The message came from outside the agent pipeline (e.g., user input).
    /// </summary>
    public static AgentRequestMessageSourceType External { get; } = new AgentRequestMessageSourceType(nameof(External));