feat: runtime state checkpointing, event system, and executor refactor

- Pass RuntimeState through the event bus and enable entity auto-registration
- Introduce checkpointing API:
  - .checkpoint(), .from_checkpoint(), and async checkpoint support
  - Provider-based storage with BaseProvider and JsonProvider
  - Mid-task resume and kickoff() integration
- Add EventRecord tracking and full event serialization with subtype preservation
- Enable checkpoint fidelity via llm_type and executor_type discriminators

- Refactor executor architecture:
  - Convert executors, tools, prompts, and TokenProcess to BaseModel
  - Introduce proper base classes with typed fields (CrewAgentExecutorMixin, BaseAgentExecutor)
  - Add generic from_checkpoint with full LLM serialization
  - Support executor back-references and resume-safe initialization

- Refactor runtime state system:
  - Move RuntimeState into state/ module with async checkpoint support
  - Add entity serialization improvements and JSON-safe round-tripping
  - Implement event scope tracking and replay for accurate resume behavior

- Improve tool and schema handling:
  - Make BaseTool fully serializable with JSON round-trip support
  - Serialize args_schema via JSON schema and dynamically reconstruct models
  - Add automatic subclass restoration via tool_type discriminator

- Enhance Flow checkpointing:
  - Support restoring execution state and subclass-aware deserialization

- Performance improvements:
  - Cache handler signature inspection
  - Optimize event emission and metadata preparation

- General cleanup:
  - Remove dead checkpoint payload structures
  - Simplify entity registration and serialization logic