Extract test configuration crud module

[akwasigroch]

Purpose

app/crud/__init__.py is the crud monolith that apps/backend/AGENTS.md says should only shrink from here. This continues the incremental split started in #2588, #2590, #2591 and #2592, taking the TestConfiguration block out.

What Changed

• New crud/test_configuration.py with get_test_configuration, get_test_configurations, create_test_configuration and update_test_configuration, moved verbatim from crud/__init__.py.
• 67 lines removed from crud/__init__.py. No re-exports left behind — the old names are gone from the monolith.
• Callers switched to direct submodule imports (from rhesis.backend.app.crud import test_configuration as test_configuration_crud): routers/test_configuration.py, services/test_run.py, services/test_set.py, jobs/execution/config.py and jobs/execution/modes.py. In services/test_set.py the call sites sit inside function-local imports, so those were adjusted in place rather than at the module header.
• Patch targets updated in tests/backend/services/test_rescore.py and tests/backend/routes/test_execute_dispatch_task_id.py.

This is a pure move: no logic, signature or behaviour changes. Import direction stays jobs → app.

Additional Context

Two pre-existing problems surfaced while tracing callers. Neither is touched here, both are worth a separate fix:

• routers/test_configuration.py:176 calls crud.delete_test_configuration, which does not exist anywhere in the codebase — that DELETE endpoint raises AttributeError at runtime. The router keeps its from rhesis.backend.app import crud import solely for that broken line.
• jobs/execution/modes.py:93 reaches a schema through the crud module as crud.schemas.TestConfigurationUpdate instead of importing schemas directly.

Two sibling PRs split other blocks of the same file in parallel. They touch different line ranges and different callers.

Testing

uv run pytest tests/backend/services/test_rescore.py tests/backend/routes/test_execute_dispatch_task_id.py tests/backend/routes/test_execute_with_reuse.py tests/backend/jobs/test_execution.py tests/backend/services/test_test_set.py tests/backend/services/test_test_execution.py → 85 passed, 3 skipped (pre-existing skips).

uv run pytest tests/backend/routes/test_quota_gates.py tests/backend/security/test_organization_filtering.py tests/backend/auth/test_object_level_auth.py tests/backend/jobs/test_validation.py tests/backend/jobs/test_filter_configs_by_scope.py → 68 passed.

Ruff check and format on all touched files are at the pre-change baseline.