fix(cluster): merge stderr with stdout in run_command

Update the `run_command` helper to support merging stderr with stdout
using the `merge_stderr` parameter. This ensures error output is captured
in the same stream, improving logging and debugging. Applied in
`start_cluster` to simplify cluster startup diagnostics.

Author: mkoura

cardano_node_tests/utils/cluster_nodes.py

@@ -392,7 +392,7 @@ def start_cluster(cmd: str, args: list[str]) -> clusterlib.ClusterLib:
     args_str = " ".join(args)
     args_str = f" {args_str}" if args_str else ""
     LOGGER.info(f"Starting cluster with `{cmd}{args_str}`.")
-    helpers.run_command(f"{cmd}{args_str}", workdir=get_cluster_env().work_dir)
+    helpers.run_command(f"{cmd}{args_str}", workdir=get_cluster_env().work_dir, merge_stderr=True)
     LOGGER.info("Cluster started.")
     return get_cluster_type().get_cluster_obj()
 

cardano_node_tests/utils/helpers.py

@@ -81,9 +81,9 @@ def run_command(
     workdir: ttypes.FileType = "",
     ignore_fail: bool = False,
     shell: bool = False,
+    merge_stderr: bool = False,
 ) -> bytes:
     """Run command."""
-    cmd: str | list
     if isinstance(command, str):
         cmd = command if shell else command.split()
         cmd_str = command
@@ -94,14 +94,17 @@ def run_command(
     LOGGER.debug("Running `%s`", cmd_str)
 
     with subprocess.Popen(
-        cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=shell, cwd=workdir or None
+        cmd,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.STDOUT if merge_stderr else subprocess.PIPE,
+        shell=shell,
+        cwd=workdir or None,
     ) as p:
         stdout, stderr = p.communicate()
         retcode = p.returncode
 
     if not ignore_fail and retcode != 0:
-        err_dec = stderr.decode()
-        err_dec = err_dec or stdout.decode()
+        err_dec = (stderr or stdout).decode()
         msg = f"An error occurred while running `{cmd_str}`: {err_dec}"
         raise RuntimeError(msg)