feat(db): refactor database backend to be database-agnostic

This allows users to choose their preferred database backend (PostgreSQL, MySQL, SQLite) while maintaining full compatibility and proper type safety.

- Replace PostgreSQL-specific JSONB with generic JSON type in models
- Implement SQLAlchemy 2.0 best practices with Mapped types and TypeDecorator
- Add configurable table name support via create_task_model() factory
- Updated metadata field mapping between Pydantic and SQLAlchemy using declared_attr
- Add comprehensive tests for metadata field mapping including complex nested data
- Update GitHub Actions to install SQL dependencies for tests
- Add pytest.importorskip to gracefully skip database tests when SQLAlchemy not installed
- Fix test_types.py to use 'id' instead of 'taskId' for JSON-RPC requests
- Add automatic cleanup of SQLite file::memory: files after tests
- Remove examples directory (moved to separate a2a-samples repo)
- Update pyproject.toml to remove workspace members reference and add `nox` for testing

Author: zboyles