Enhance TestAgent Durable Object with Initialization and State Management

- Updated TestAgent to include state management and initialization logic for handling test execution sessions.
- Implemented SQL database setup for storing agent actions and evidence.
- Added error handling and user-friendly responses for various endpoints.
- Introduced integration tests for core functionalities of the TestAgent.
- Updated wrangler.toml to reflect changes in class definitions and migration settings.

This commit addresses Story 2.1: Full implementation of the TestAgent Durable Object.

Author: adam0white