Reconsider IA2AClient, ITaskManager, ITaskStore interfaces

### Context:
The A2A SDK declares a few public interfaces: `IA2AClient`, `ITaskManager`, `ITaskStore`, and corresponding classes that implement them. While having the interfaces may be quite convenient for polymorphic (when different implementations are required) and testability (mocking a class for testing purposes) cases, exposing the interfaces as part of the public API surface of SDKs may have downsides, such as a source of breaking changes for all third-party implementations that SDK authors are not aware of when new members are added to them.

### ToDo:
1. Identify all public interfaces in the SDK.
2. For each interface, consider:
- Whether it is actually required, and if not, consider removing it.
- Whether it can be turned into an abstract class. New members can easily be added to abstract classes as virtual without breaking existing implementations, but there could be a downside in case a consumer's class already inherits from another class - C# does not allow multiple inheritance.

Start by locating the public interfaces in the SDK codebase, likely in a directory like src/A2A. Examine IA2AClient, ITaskManager, ITaskStore and any others. Review their usage in the project and tests to understand if they are required. Consider the trade-offs between interfaces and abstract classes for each, and check for any existing third-party implementations. The outcome is a decision on whether to remove, keep, or convert each interface.

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