test: reconcile master's unit tests with feat/sandbox refactors

The merge from master brought in new unit tests that target pre-refactor
APIs on feat/sandbox. Reconcile each:

- factories/app.py: FakeApp now exposes a Mock skill_mgr (with empty .skills
  dict + inert prompt-addition builder) and a Mock pipeline_service so the
  PreProcessor skill-index injection branch can run end-to-end in tests.

- pipeline/conftest.py: eagerly import langbot.pkg.pipeline.pipelinemgr so
  pipeline.stage is fully initialised before any individual stage test
  (preproc, longtext, ...) tries to lazy-load it. Without this preload,
  running test_preproc.py in isolation hit a circular-import error via the
  stage -> app -> pipelinemgr -> stage chain.

- provider/test_tool_manager.py: ToolManager now probes four loaders
  (native -> plugin -> mcp -> skill). Inject inert native + skill mocks in
  the execute_func_call fixture and assert all four shutdowns fire.

- utils/test_paths.py: drop the three cwd-dependent _check_if_source_install
  cases. The refactor walks Path(__file__).resolve().parents looking for
  pyproject.toml + main.py, so cwd no longer factors in and there's no
  file read to mock-fail. The positive case and caching test still apply.

- utils/test_version.py: delete entirely. is_newer and compare_version_str
  were removed when VersionManager was refactored to use the Space API for
  release checks (1b4107a9); the tests targeted a surface that no longer
  exists.

tests/unit_tests/pipeline/conftest.py

@@ -12,6 +12,12 @@ from __future__ import annotations
 import pytest
 from unittest.mock import AsyncMock, Mock
 
+# Preload pipelinemgr so the pipeline.stage module is fully initialised before
+# any individual stage test (e.g. preproc, longtext) tries to import it. Without
+# this, running a stage test in isolation triggers a circular-import error:
+#   stage.py → core.app → pipelinemgr → stage.stage_class (not yet bound).
+import langbot.pkg.pipeline.pipelinemgr  # noqa: F401
+
 import langbot_plugin.api.entities.builtin.pipeline.query as pipeline_query
 import langbot_plugin.api.entities.builtin.platform.message as platform_message
 import langbot_plugin.api.entities.builtin.platform.events as platform_events