.NET: Make DelegatingAIAgent abstract

[Copilot]

Motivation and Context

DelegatingAIAgent implements the decorator pattern as a base class for composable agent pipelines. It was designed to be inherited, not instantiated directly - it has a protected constructor and only provides pass-through behavior. Making it abstract enforces this design intent.

Contributes to: #2542

Description

Changed the class declaration from public class to public abstract class:

public abstract class DelegatingAIAgent : AIAgent
{
    protected DelegatingAIAgent(AIAgent innerAgent)
    {
        this.InnerAgent = Throw.IfNull(innerAgent);
    }
    // ...
}

This prevents direct instantiation while maintaining full compatibility with all inheriting classes: AnonymousDelegatingAIAgent, LoggingAgent, OpenTelemetryAgent, EntityAgentWrapper, and various sample implementations.

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.

Original prompt

Make DelegatingAIAgent abstract

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[Copilot]

Pull request overview

This PR makes the DelegatingAIAgent class abstract to enforce its intended design as a base class for the decorator pattern. The class was always designed to be inherited (protected constructor, pass-through behavior), and making it abstract prevents direct instantiation while maintaining full compatibility with all existing inheritors.

• Changed class declaration from public class to public abstract class