fix(workspace): register plugins_registry as sys.modules shim before loading adapters

KI-296 fix: when the PyPI-installed runtime (molecule-ai-workspace-runtime
0.1.129+) ships plugins_registry as molecule_runtime.plugins_registry (a
subpackage), plugin adapter files that do ``from plugins_registry import ...``
as a top-level name fail with ModuleNotFoundError because Python's import
system cannot find a top-level ``plugins_registry`` package.

The fix in plugins_registry/__init__.py:_load_module_from_path() registers
molecule_runtime.plugins_registry as ``plugins_registry`` in sys.modules
before exec'ing any plugin adapter file, so the top-level import resolves
correctly in both environments:
- PyPI wheel (molecule_runtime.plugins_registry → sys.modules["plugins_registry"])
- molecule-core workspace source (top-level workspace/plugins_registry already
  on sys.path; the setdefault is a no-op)

Submodules (builtins, protocol, raw_drop) are also registered so adapters
that import ``from plugins_registry.builtins import ...`` work without error.

Added test_load_module_from_path_registers_plugins_registry_sys_modules.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workspace/plugins_registry/__init__.py

@@ -51,6 +51,32 @@ class AdaptorSource:
 
 def _load_module_from_path(module_name: str, path: Path):
     """Import a Python file by absolute path. Returns the module or None on failure."""
+
+    # KI-296: Before exec'ing plugin-adapter files (which import
+    # ``from plugins_registry import ...`` as a top-level name), register
+    # the molecule-runtime subpackage as ``plugins_registry`` in sys.modules.
+    # In the molecule-core workspace source this is already a top-level package,
+    # so the setdefault is a no-op. In the PyPI-installed runtime wheel
+    # (molecule-ai-workspace-runtime 0.1.129+), the package ships as
+    # ``molecule_runtime.plugins_registry`` and without this shim every
+    # plugin adapter would fail with ModuleNotFoundError.
+    import sys as _sys
+
+    if "plugins_registry" not in _sys.modules:
+        try:
+            _mr_pr = __import__("molecule_runtime.plugins_registry", fromlist=[""])
+            _sys.modules["plugins_registry"] = _mr_pr
+            # Also register submodules the adapters commonly import directly.
+            for _sub in ("builtins", "protocol", "raw_drop"):
+                _submod = getattr(_mr_pr, _sub, None)
+                if _submod is not None:
+                    _sys.modules[f"plugins_registry.{_sub}"] = _submod
+        except ImportError:
+            # molecule-runtime not installed (e.g. test environment with
+            # workspace/ on sys.path directly) — skip shim; the top-level
+            # workspace/plugins_registry package is already findable.
+            pass
+
     spec = importlib.util.spec_from_file_location(module_name, path)
     if spec is None or spec.loader is None:
         return None

workspace/tests/test_plugins_registry.py

@@ -325,3 +325,44 @@ def test_resolve_registry_missing_module_falls_through(monkeypatch, tmp_path: Pa
     monkeypatch.setattr(pr, "_REGISTRY_ROOT", tmp_path / "empty-registry")
     _, source = pr.resolve("demo-plugin", "test_runtime", plugin_root)
     assert source == AdaptorSource.RAW_DROP
+
+
+def test_load_module_from_path_registers_plugins_registry_sys_modules(tmp_path: Path):
+    """KI-296: _load_module_from_path registers ``plugins_registry`` in sys.modules
+    before exec'ing the adapter, so adapter files that do
+    ``from plugins_registry import ...`` resolve correctly when the runtime is
+    installed from the PyPI wheel (where the package ships as
+    ``molecule_runtime.plugins_registry`` rather than a top-level ``plugins_registry``).
+    """
+    import sys as _sys
+    import plugins_registry as pr
+
+    # Create a fake adapter that imports plugins_registry at top level.
+    adapter_file = tmp_path / "fake_runtime_adapter.py"
+    adapter_file.write_text(
+        "from plugins_registry import InstallContext  # noqa: F401\n"
+        "from plugins_registry.builtins import AgentskillsAdaptor as Adaptor  # noqa: F401\n"
+    )
+
+    # Evict any pre-existing sys.modules entries for the shim keys so the
+    # import inside _load_module_from_path actually runs.
+    _saved = {
+        k: _sys.modules.pop(k, None)
+        for k in (
+            "plugins_registry", "plugins_registry.builtins",
+            "plugins_registry.protocol", "plugins_registry.raw_drop",
+            "_plugin_adaptor.test.fake_runtime",
+        )
+    }
+
+    try:
+        result = pr._load_module_from_path("_plugin_adaptor.test.fake_runtime", adapter_file)
+        assert result is not None, "module should load without ImportError"
+        assert hasattr(result, "Adaptor"), "AgentskillsAdaptor alias should be in namespace"
+    finally:
+        # Restore sys.modules state.
+        for k, v in _saved.items():
+            if v is None:
+                _sys.modules.pop(k, None)
+            else:
+                _sys.modules[k] = v