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/pipeline/test_n8nsvapi.py

@@ -14,24 +14,40 @@ import json
 import sys
 from unittest.mock import AsyncMock, MagicMock, Mock, patch
 
-# Break the circular import chain before importing n8nsvapi:
-#   n8nsvapi → runner → app → pipelinemgr → all runners → runner (partially init)
-_mock_runner = MagicMock()
-_mock_runner.runner_class = lambda name: (lambda cls: cls)  # no-op decorator
-_mock_runner.RequestRunner = object
-sys.modules.setdefault('langbot.pkg.provider.runner', _mock_runner)
-sys.modules.setdefault('langbot.pkg.core.app', MagicMock())
-sys.modules.setdefault('langbot.pkg.utils.httpclient', MagicMock())
-
 import pytest
 import langbot_plugin.api.entities.builtin.provider.message as provider_message
-from langbot.pkg.provider.runners.n8nsvapi import N8nServiceAPIRunner
+
+# Break the circular import chain while importing n8nsvapi:
+#   n8nsvapi → runner → app → pipelinemgr → all runners → runner (partially init)
+# The stubs are restored immediately afterwards so this module does NOT pollute
+# sys.modules for other test modules (e.g. ones importing the real
+# LocalAgentRunner, which would otherwise inherit ``object`` and break).
+_runner_stub = MagicMock()
+_runner_stub.runner_class = lambda name: (lambda cls: cls)  # no-op decorator
+_runner_stub.RequestRunner = object
+_import_stubs = {
+    'langbot.pkg.provider.runner': _runner_stub,
+    'langbot.pkg.core.app': MagicMock(),
+    'langbot.pkg.utils.httpclient': MagicMock(),
+}
+_saved_modules = {name: sys.modules.get(name) for name in _import_stubs}
+for _name, _stub in _import_stubs.items():
+    sys.modules[_name] = _stub
+try:
+    from langbot.pkg.provider.runners.n8nsvapi import N8nServiceAPIRunner
+finally:
+    for _name, _original in _saved_modules.items():
+        if _original is None:
+            sys.modules.pop(_name, None)
+        else:
+            sys.modules[_name] = _original
 
 
 # ---------------------------------------------------------------------------
 # Helpers
 # ---------------------------------------------------------------------------
 
+
 def make_runner(output_key: str = 'response') -> N8nServiceAPIRunner:
     ap = Mock()
     ap.logger = Mock()
@@ -74,6 +90,7 @@ async def collect_chunks(runner: N8nServiceAPIRunner, chunks: list[bytes | str])
 # _process_response: stream format (type:item/end)
 # ---------------------------------------------------------------------------
 
+
 @pytest.mark.asyncio
 async def test_stream_format_single_item():
     """Single item + end in one chunk yields final chunk with full content."""
@@ -153,6 +170,7 @@ async def test_stream_format_no_spurious_empty_yield():
 # _process_response: plain JSON fallback
 # ---------------------------------------------------------------------------
 
+
 @pytest.mark.asyncio
 async def test_plain_json_with_output_key():
     """Plain JSON with matching output_key extracts value via output_key."""
@@ -223,6 +241,7 @@ async def test_invalid_json_returns_raw_text():
 # _call_webhook: output type depends on is_stream
 # ---------------------------------------------------------------------------
 
+
 def make_query(is_stream: bool):
     """Build a minimal Query mock."""
     query = Mock()