fix: ImportExportServersTestCase uses sys.executable + surfaces subprocess errors

Two latent bugs in pgadmin/setup/tests/test_export_import_servers.py
caused this test to fail on macOS / modern Linux distros:

1. The test invoked the subprocesses with a hardcoded "python" via
   os.system. macOS and many modern Linux distros do not provide a
   "python" alias — only "python3" or a venv-specific binary. The
   subprocess fails with "sh: python: command not found", exits non-
   zero, and writes nothing to the dump-servers output file.

   Same root cause that a50a553b0 ("chore: feature tests use
   sys.executable") fixed for AppStarter; this is the same fix in the
   regression test.

2. Both os.system calls redirected stderr to /dev/null. When the
   subprocess failed the empty output file then tripped json.loads
   with the misleading "Expecting value: line 1 column 1 (char 0)"
   instead of surfacing the underlying "command not found" error.

Switch to subprocess.run with a list (so there is no shell quoting and
no command-injection surface), use sys.executable, capture stderr, and
self.fail() with the captured stderr if the subprocess exits non-zero.

Verified: setup.tests goes from 4 pass / 1 fail / 1 skip to 5 pass / 0
fail / 1 skip on PG18.

web/pgadmin/setup/tests/test_export_import_servers.py

@@ -10,6 +10,8 @@
 from pgadmin.utils.route import BaseTestGenerator
 import os
 import json
+import subprocess
+import sys
 import tempfile
 import config
 
@@ -34,16 +36,37 @@ class ImportExportServersTestCase(BaseTestGenerator):
         path = os.path.dirname(__file__)
         setup = os.path.realpath(os.path.join(path, "../../../setup.py"))
 
-        # Load the servers
-        os.system(
-            "python \"%s\" load-servers \"%s\" 2> %s" %
-            (setup, os.path.join(path, "servers.json"), os.devnull)
+        # Use sys.executable instead of bare "python" — macOS / many Linux
+        # distros do not provide a "python" alias, only "python3" or a venv-
+        # specific binary. Use subprocess.run with a list so there is no
+        # shell quoting and no command injection surface.  Surface the
+        # subprocess error directly instead of letting a silent failure
+        # produce an empty file that downstream json.loads then misreports
+        # as "Expecting value: line 1 column 1".
+        load_result = subprocess.run(
+            [sys.executable, setup, "load-servers",
+             os.path.join(path, "servers.json")],
+            capture_output=True,
+            text=True,
+            check=False,
         )
+        if load_result.returncode != 0:
+            self.fail(
+                "load-servers exited {0}: {1}".format(
+                    load_result.returncode, load_result.stderr))
 
         # And dump them again
         tf = tempfile.NamedTemporaryFile(delete=False)
-        os.system("python \"%s\" dump-servers \"%s\" 2> %s" %
-                  (setup, tf.name, os.devnull))
+        dump_result = subprocess.run(
+            [sys.executable, setup, "dump-servers", tf.name],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        if dump_result.returncode != 0:
+            self.fail(
+                "dump-servers exited {0}: {1}".format(
+                    dump_result.returncode, dump_result.stderr))
 
         # Compare the JSON files, ignoring servers that exist in our
         # generated file but not the test config (they are likely