[M2] Implement core A2A domain models (Task, Message, Part, Artifact, AgentCard)

[rdobrik]

Description

Replace minimal request-only message model with typed A2A domain models matching latest protocol structures.

Tasks

• Add models for Task, TaskStatus, Message, Part, Artifact.
• Add models for AgentCard and capabilities/security metadata.
• Add request/response DTOs for core methods.
• Add serialization/deserialization tests for all models.

Acceptance Criteria

• Models map cleanly to JSON payload examples.
• Backward-incompatible fields are intentionally versioned/documented.
• Unit tests cover round-trip serialization.

Dependencies

• Issue 1