test: repair stale skill/sandbox tests for feat/sandbox

The skill subsystem moved to Tool-Call activation and a Box-managed
skill store; several tests still asserted removed APIs and a sys.modules
stub leaked across the suite. Full unit suite now green (was 23 failing).

- test_skill_tools: drop TestSkillManagerActivation (text-marker API
  removed); rewrite TestSkillActivationHelper around the current
  skill.activation.register_activated_skill; replace the CRUD
  TestSkillAuthoringToolLoader with TestSkillToolLoader covering the
  current activate/register_skill tools and sandbox-availability gating
- test_tool_manager_native: ToolManager attr is skill_tool_loader (not
  skill_authoring_tool_loader); native loader now exposes 6 tools
  (exec/read/write/edit/glob/grep) and requires initialize() with a
  backend-available get_status()
- test_localagent_sandbox_exec: remove obsolete activation-marker
  leakage tests and their helper providers
- test_model_service / pipeline conftest: give the mocks skill_mgr=None
  so PreProcessor's local-agent skill-binding guard short-circuits
- test_n8nsvapi: stop permanently overwriting sys.modules
  ('langbot.pkg.provider.runner' etc.); save and restore around the
  import so other modules get the real LocalAgentRunner base class

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit_tests/provider/test_model_service.py

@@ -69,6 +69,7 @@ async def test_updated_llm_model_is_immediately_usable_by_local_agent_pipeline()
     ap.logger = Mock()
     ap.persistence_mgr = SimpleNamespace(execute_async=AsyncMock())
     ap.tool_mgr = SimpleNamespace(get_all_tools=AsyncMock(return_value=[]))
+    ap.skill_mgr = None  # PreProcessor only uses skill_mgr for the local-agent skill-binding branch
     ap.plugin_connector = SimpleNamespace(
         emit_event=AsyncMock(return_value=SimpleNamespace(event=SimpleNamespace(default_prompt=[], prompt=[])))
     )