fix(BA-3307): Remove fk relationship rows(session, route) when purge scaling group

[seedspirit]

resolves #7202 (BA-3307)

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[Copilot]

Pull request overview

This PR fixes BA-3307 by implementing proper cascade deletion of foreign key relationships (sessions and routes) when purging a scaling group. The fix prevents integrity errors that previously occurred when attempting to delete a scaling group that had associated sessions or routes.

Key Changes:

• Implements a new purge_scaling_group action and repository method that handles cascade deletion in the correct order: RoutingRow → SessionRow → ScalingGroupRow
• Migrates the GraphQL DeleteScalingGroup mutation from direct SQL deletion to using the new purge action
• Adds test coverage for the purge operation with basic scenarios

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/manager/repositories/scaling_group/test_scaling_group_repository.py	Adds tests for purge_scaling_group operation including success and not-found scenarios
src/ai/backend/manager/services/scaling_group/service.py	Adds service method to handle purge_scaling_group action
src/ai/backend/manager/services/scaling_group/processors.py	Registers purge_scaling_group action processor
src/ai/backend/manager/services/scaling_group/actions/purge_scaling_group.py	Defines new PurgeScalingGroupAction and result classes
src/ai/backend/manager/repositories/scaling_group/repository.py	Adds repository method for purging scaling groups
src/ai/backend/manager/repositories/scaling_group/db_source/db_source.py	Implements cascade deletion logic: routes → sessions → scaling group
src/ai/backend/manager/models/gql_models/scaling_group.py	Refactors DeleteScalingGroup mutation to use new purge action, removes old error handling
changes/7582.fix.md	Documents the fix in changelog

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