fix(BA-4282): Remove kernel row when purge scaling group preventing fk error

[seedspirit]

resolves #8613 (BA-4282)

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 pull request fixes a foreign key constraint violation (IntegrityError) that occurs when purging a scaling group that has sessions with associated kernels. The issue was that the cascade deletion sequence was attempting to delete sessions before deleting their associated kernels, which violates the fk_kernels_session_id_sessions foreign key constraint (which uses the default RESTRICT behavior).

Changes:

• Added kernel deletion step in the cascade deletion sequence for purge_scaling_group
• Added comprehensive test to verify purging scaling groups with the full FK hierarchy (scaling group → session → kernel + endpoint → route)

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/ai/backend/manager/repositories/scaling_group/db_source/db_source.py	Added deletion of KernelRow records between endpoint and session deletion to prevent FK constraint violations
tests/unit/manager/repositories/scaling_group/test_scaling_group_repository.py	Added test fixtures and test case to verify purging works correctly with the full entity hierarchy including kernels

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