Fix lift helper crash on model-level pydantic errors

`_navigable_path` unpacked `*prefix, leaf = loc` and crashed on
empty loc tuples, which pydantic emits for `@model_validator`
errors. Return early with no path components instead.

Also fix `test_lift_handles_nested_conditional_drift` to not
inherit the BASE_TOOL `shell_command` reference to `inputs.msg`,
since the test's conditional-only input set doesn't declare it
and the `_check_input_refs_and_outputs` validator (added after
the test) now rejects it.

Author: mvdbeek

lib/galaxy/tool_util_models/__init__.py

@@ -279,6 +279,8 @@ def _navigable_path(value: Any, loc: tuple) -> Tuple[Optional[Any], List[Any]]:
     container of the leaf and the cleaned path components, or (None, []) if
     the path can't be resolved."""
     cur: Any = value
+    if not loc:
+        return cur, []
     *prefix, leaf = loc
     cleaned: List[Any] = []
     for step in prefix:

test/unit/tool_util_models/test_user_tool_source_response.py

@@ -86,6 +86,7 @@ def test_lift_status_lifted_for_drift_only():
 def test_lift_handles_nested_conditional_drift():
     value = {
         **BASE_TOOL,
+        "shell_command": "echo hello",
         "inputs": [
             {
                 "type": "conditional",