refactor(steering): remove legacy steering handlers in favor of unified framework

BREAKING CHANGE: Legacy steering handlers have been removed.
Unified steering is now the only implementation.

Removed files:
- src/core/services/tool_call_handlers/inline_python_steering_handler.py
- src/core/services/tool_call_handlers/pytest_full_suite_handler.py
- src/core/services/tool_call_handlers/config_steering_handler.py
- tests/unit/core/services/tool_call_handlers/test_inline_python_steering_handler.py
- tests/unit/core/services/tool_call_handlers/test_pytest_full_suite_handler.py
- tests/unit/core/services/tool_call_handlers/test_apply_diff_handler.py
- tests/unit/tool_call_handlers/test_pytest_full_suite_handler.py

Updated files:
- src/core/services/tool_call_handlers/__init__.py: Removed exports
- src/core/di/services.py: Removed legacy handler registration logic
- src/core/config/app_config.py: Deprecated unified_steering_enabled flag
- src/core/services/tool_call_handlers/pytest_context_saving_handler.py:
  Updated imports to use unified policy
- tests/integration/conftest.py: Simplified fixture

The unified steering framework (src/services/steering/) now handles all
steering functionality with policies for:
- InlinePythonPolicy
- PytestFullSuitePolicy
- ConfiguredRulesPolicy

Configuration options remain compatible with the existing API.

Author: Mateusz