Summary & Motivation

This PR creates a structured mixins architecture for DagsterInstance to improve code organization and maintainability. The massive DagsterInstance class (over 1400 lines) is reorganized by extracting functionality into focused mixin classes.

Key changes:

• Create mixins package: New python_modules/dagster/dagster/_core/instance/mixins/ directory for organized mixin classes
• Move domains_mixin: Relocate existing domains_mixin.py to the new mixins package with improved type safety using cast()
• Extract RunsMixin: Move 300+ lines of runs-related functionality from DagsterInstance into dedicated RunsMixin, including:
  • Run retrieval methods (get_run_by_id, get_run_record_by_id)
  • Run creation methods (create_run, create_run_for_job)
  • Run lifecycle management (add_run, delete_run, handle_run_event)
  • Run launcher operations (launch_run, submit_run, resume_run)
  • Run reporting (report_run_canceling, report_run_failed)
• Create SettingsMixin: Extract settings and configuration-related functionality
• Update inheritance: DagsterInstance now inherits from SettingsMixin, RunsMixin, DynamicPartitionsStore, and DomainsMixin
• Improve imports: Update all import paths to use new mixins package structure

The mixins follow the established architectural pattern where simple implementations delegate to domain objects for complex business logic, maintaining clean separation of concerns while improving code organization.

How I Tested These Changes

Verified that all imports resolve correctly and existing functionality is preserved through the delegation pattern. The reorganization maintains the same public API surface area.