fix(run): review nits — module_dep self-append, surface test stderr

The else-branch of the mpi4py dependency line appended the whole list
to itself (harmless only because DependencyHandler dedups); with the
launcher gate this became the common path. And the regression test now
surfaces the subprocess stderr on failure instead of swallowing it in
a bare CalledProcessError.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: cailmdaley

src/shapepipe/run.py

@@ -194,7 +194,7 @@ def _check_dependencies(self):
         module_dep = self._get_module_depends("depends") + __installs__
         module_exe = self._get_module_depends("executes")
 
-        module_dep += ["mpi4py"] if import_mpi else module_dep
+        module_dep += ["mpi4py"] if import_mpi else []
 
         exe_to_module = {
             exe: module

src/shapepipe/tests/test_run.py

@@ -36,7 +36,9 @@ def _import_mpi_flag(extra_env):
         env=env,
         capture_output=True,
         text=True,
-        check=True,
+    )
+    assert result.returncode == 0, (
+        f"subprocess failed (exit {result.returncode}): {result.stderr}"
     )
     return result.stdout.strip()