Add support for custom HTTP tools in scenario tool resolution and validation:
- Extend resolved_tools to load custom tool metadata from database
- Add resolve_tool_instance method to instantiate HttpTool for custom tools
- Update validation to include account-scoped custom tools in available tools
- Support mixed static and custom tool references in scenarios
- Filter out disabled custom tools at resolution time
Test coverage includes validation, resolution, and execution paths for
custom tools with account isolation and enable/disable states.