tech-debt(backend): ConfigManager.get_section AttributeError breaks 6 config/multimodal integration tests #12722

What

Discovered while verifying PR #12721 (issue #12714, torch-loader
unification) — pre-existing, unrelated to that change (confirmed via
`git diff origin/Dev_new_gui` showing 0 changes to any of these test
files or to `config/manager.py`).

6 tests in `autobot-backend/config/config_migration_integration_test.py`
fail locally:

```
FAILED ...::test_config_environment_variable_priorities - AssertionError: assert None == 'autobot_value'
FAILED ...::test_unified_multimodal_processor_config_usage - AttributeError: 'ConfigManager' object has no attribute 'get_section'
FAILED ...::test_config_section_completeness - AttributeError: 'ConfigManager' object has no attribute 'get_section'
FAILED ...::test_config_type_consistency - AssertionError: assert None is not None (path: 'multimodal.vision.enabled')
FAILED ...::test_config_migration_backward_compatibility - ModuleNotFoundError: No module named 'utils.config_manager'
FAILED ...::test_config_default_value_handling - AssertionError: assert None in ['ollama', 'openai']
```

Plus the same `ConfigManager` object has no attribute `get_section``
failure recurs in:

• `autobot-backend/multimodal_processor/multimodal_system_integration_test.py`
  (7 errors — `VisionProcessor()` setup calls `get_config_section`, which
  the test's mock wires to `mock_config.get_section`, but the fixture's
  `ConfigManager` instance has no such method)
• `autobot-backend/system_benchmarks_performance_test.py` (multiple
  failures downstream of the same fixture pattern)

Likely root cause

`get_section` exists on `config.registry.ConfigRegistry`
(`autobot-backend/config/registry.py:161`), not on
`config.manager.ConfigManager`. These test fixtures appear to predate a
`ConfigManager`/`ConfigRegistry` split (or a naming-cleanup pass) and
were never updated to call the right class, or `ConfigManager` is
missing a `get_section` passthrough it's expected to have.

`test_config_migration_backward_compatibility` additionally imports
`utils.config_manager`, a module that no longer exists.

Reproduce

```
cd autobot-backend
python3 -m pytest config/config_migration_integration_test.py multimodal_processor/multimodal_system_integration_test.py system_benchmarks_performance_test.py -q
```

Scope

Out of scope for #12714/PR #12721 (pure config-subsystem test drift, no
relation to the torch lazy-loader unification). Needs its own
investigation: either add `ConfigManager.get_section` as a real method/
passthrough to `ConfigRegistry.get_section`, or fix the test fixtures to
call the correct class, plus remove/redirect the dead
`utils.config_manager` import.