.NET: Expose metadata from A2AAgent and seal AIAgentMetadata

[westey-m]

Motivation and Context

#2542

Description

• Expose metadata from A2AAgent and seal AIAgentMetadata

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 exposes metadata from the A2AAgent class and seals the AIAgentMetadata class as part of the .NET Single Agent API review (issue #2542).

Changes:

• Sealed the AIAgentMetadata class to prevent inheritance
• Added GetService method override to A2AAgent to expose A2AClient and AIAgentMetadata
• Added comprehensive unit tests for the GetService method in A2AAgent

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AIAgentMetadata.cs	Sealed the AIAgentMetadata class to prevent inheritance
dotnet/src/Microsoft.Agents.AI.A2A/A2AAgent.cs	Added GetService override to expose A2AClient and AIAgentMetadata with static metadata instance
dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/A2AAgentTests.cs	Added comprehensive unit tests for GetService method covering all code paths