feat: seal api and clean up a2aclient tests

[SergeyMenshykh]

This PR seals public API and model classes, makes some of them abstract, and cleans up the unit tests for A2AClient.

[gemini-code-assist]

Summary of Changes

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This pull request focuses on enhancing the robustness and clarity of the public API by explicitly defining class extensibility through the use of sealed and abstract keywords. Concurrently, it improves the maintainability of the A2AClient's unit tests by streamlining their setup and reducing duplication.

Highlights

• API Design - Sealing Classes: A significant number of public API and model classes across A2A/JsonRpc, A2A/Models, and A2A/Server directories have been marked as sealed. This prevents external code from inheriting from these classes, promoting API stability and preventing unintended extensions.
• API Design - Abstracting Base Classes: The A2AEvent and A2AResponse base classes in src/A2A/Models/A2AResponse.cs have been changed from concrete classes to abstract. This clarifies their role as base types that cannot be directly instantiated, enforcing proper inheritance patterns for derived event and response types.
• Test Suite Refactoring: The unit tests for A2AClient in tests/A2A.UnitTests/Client/A2AClientTests.cs have undergone a substantial cleanup. The CreateA2AClient helper method has been refactored to centralize the mock HTTP client setup, reducing boilerplate and improving the readability and maintainability of the tests, especially for handling different response types (e.g., standard JSON-RPC vs. SSE).

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[gemini-code-assist]

Code Review

The pull request seals several classes and makes A2AEvent and A2AResponse abstract. It also cleans up the unit tests for A2AClient by using concrete types in CreateA2AClient.