refactor(BA-3117, BA-3118): Introduce source-based structure in user resource policy

[seedspirit]

resolves #6888, #6889 (BA-3117, BA-3118)

Apply the DB Source to the Repository within the User resource policy. The DB Source communicates with the actual database, fetching data that meets specified conditions and returning DTOs or Entities to the repository. This enables the Repository layer to combine various Sources without needing to know the detailed implementation of the DB, thereby returning data suitable for the Service layer.

This PR includes the introduction of the DB Source and the resulting modifications to the test code.
As the Repository's current role is not significant, tests communicating with the actual database are conducted within the Repository tests.
The existing integration tests overlap with the newly written tests and are less aligned with Backend.AI's test code writing patterns, so we intend to remove them.

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

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📚 Documentation preview 📚: https://sorna--6907.org.readthedocs.build/en/6907/

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📚 Documentation preview 📚: https://sorna-ko--6907.org.readthedocs.build/ko/6907/

[Copilot]

Pull request overview

This PR refactors the user resource policy repository to introduce a source-based architecture pattern by adding a DB Source layer that handles database operations. The repository layer now delegates to this DB source, improving separation of concerns and maintainability.

Key changes:

• Introduced UserResourcePolicyDBSource to encapsulate database operations
• Refactored UserResourcePolicyRepository to delegate to the new DB source
• Updated tests from mocked database tests to real database integration tests in the repository layer
• Replaced generic ObjectNotFound exception with domain-specific UserResourcePolicyNotFound

Reviewed changes

Copilot reviewed 15 out of 16 changed files in this pull request and generated 13 comments.

Show a summary per file

File	Description
src/ai/backend/manager/repositories/user_resource_policy/db_source/db_source.py	New DB source class implementing database operations for user resource policies
src/ai/backend/manager/repositories/user_resource_policy/repository.py	Refactored to delegate CRUD operations to the DB source layer
src/ai/backend/manager/services/user_resource_policy/service.py	Simplified to pass creator/modifier objects directly to repository
src/ai/backend/manager/models/resource_policy.py	Added from_creator classmethod to construct row from creator object
src/ai/backend/common/exception.py	Added domain-specific UserResourcePolicyNotFound exception
src/ai/backend/common/metrics/metric.py	Added USER_RESOURCE_POLICY_DB_SOURCE layer type for metrics
tests/manager/repositories/user_resource_policy/test_user_resource_policy_repository.py	Migrated from mock-based tests to real database integration tests
tests/manager/services/user_resource_policy/test_user_resource_policy.py	Updated assertions to verify creator/modifier objects are passed correctly
tests/manager/integration/services/user_resource_policy/*	Removed redundant integration tests that overlap with new repository tests
docs/manager/graphql-reference/*.graphql	GraphQL schema changes (unrelated to this PR)
changes/6907.enhance.md	Changelog entry for this enhancement

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[HyeockJinKim]

Why is serializable applied here? It seems like applying Read Committed this time would be worthwhile, don't you think?

[HyeockJinKim]

It seems like applying it in 1.4 would be limited... Let's see after it goes over 2.0 for now.