.NET: Update AgentFeatureCollections with feedback

[westey-m]

Motivation and Context

#2325

Description

• Adding Remove
• Switching to TryGet from Get
• Removing indexer
• Requiring all features to be non-null
• Adding class restriction to TFeature

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 updates the AgentFeatureCollection API based on feedback from issue #2325. The changes improve the API design by replacing the indexer with a TryGet method, requiring non-null features, and adding a Remove method.

Key changes:

• Replaced Get<TFeature>() method with TryGet<TFeature>(out TFeature feature) for safer feature retrieval
• Removed the indexer this[Type key] in favor of explicit Set, TryGet, and Remove methods
• Added Remove<TFeature>() method to explicitly remove features from the collection
• Added class constraint to all generic type parameters and made all features non-nullable
• Added WithFeature extension method for fluent API usage
• Updated all usages across the codebase to use the new API

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/Features/IAgentFeatureCollection.cs	Updated interface with TryGet, Remove, and Set methods, removed indexer
dotnet/src/Microsoft.Agents.AI.Abstractions/Features/AgentFeatureCollection.cs	Implemented new API methods, renamed _defaults to _innerCollection for clarity
dotnet/src/Microsoft.Agents.AI.Abstractions/Features/AgentFeatureCollectionExtensions.cs	Added new extension methods file with WithFeature helper
dotnet/src/Microsoft.Agents.AI.Abstractions/Features/ConversationIdAgentFeature.cs	Updated documentation to remove reference to 3rd party stores
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Updated to use TryGet pattern instead of Get
dotnet/src/Microsoft.Agents.AI.CopilotStudio/CopilotStudioAgent.cs	Updated to use TryGet pattern instead of Get
dotnet/src/Microsoft.Agents.AI.A2A/A2AAgent.cs	Updated to use TryGet pattern instead of Get
dotnet/samples/GettingStarted/Agents/Agent_Step07_3rdPartyThreadStorage/Program.cs	Updated sample to use new WithFeature extension method
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentFeatureCollectionTests.cs	Updated tests to cover new API methods including Remove and TryGet

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated no new comments.

[stephentoub]

I see AgentRunOptions now has both AdditionalProperties and Features:

agent-framework/dotnet/src/Microsoft.Agents.AI.Abstractions/AgentRunOptions.cs

Lines 93 to 98 in 5b39389

	public AdditionalPropertiesDictionary? AdditionalProperties { get; set; }
	/// <summary>
	/// Gets or sets the collection of features provided by the caller and middleware for this run.
	/// </summary>
	public IAgentFeatureCollection? Features { get; set; }

What's the use case for having both? When would I pick one over the other? Do we need Features if AdditionalProperties exists, or vice versa?

[westey-m]

I see AgentRunOptions now has both AdditionalProperties and Features:

agent-framework/dotnet/src/Microsoft.Agents.AI.Abstractions/AgentRunOptions.cs

Lines 93 to 98 in 5b39389

	public AdditionalPropertiesDictionary? AdditionalProperties { get; set; }
	/// <summary>
	/// Gets or sets the collection of features provided by the caller and middleware for this run.
	/// </summary>
	public IAgentFeatureCollection? Features { get; set; }

What's the use case for having both? When would I pick one over the other? Do we need Features if AdditionalProperties exists, or vice versa?

@stephentoub, yeah, this still bugs me as well. Let's discuss directly offline. I don't intend to merge the feature branch until we have made a decision on this.
CC @ReubenBond

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 7 comments.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

[ReubenBond]

Just one comment about the indexer removal