feat: async subprocess in nagios_runner with stderr capture and signal handling

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_nagios_runner.py

@@ -20,3 +20,43 @@ def test_no_commands_sets_skip_reason():
     assert result is False
     assert plugin.skip_reason is not None
     assert "nagios_runner.commands" in plugin.skip_reason
+
+
+def test_stderr_used_when_stdout_empty(tmp_path):
+    script = tmp_path / "check_err.sh"
+    script.write_text("#!/bin/sh\necho 'error from stderr' >&2\nexit 2\n")
+    script.chmod(script.stat().st_mode | stat.S_IEXEC)
+
+    config = {"commands": [{"name": "t", "command": str(script)}], "timeout": 5}
+    plugin = NagiosRunnerPlugin(config=config)
+    asyncio.run(plugin.initialize())
+    data = asyncio.run(plugin._collect_metrics())
+
+    assert "error from stderr" in data["t_output"]
+    assert data["t_status_code"] == NAGIOS_CRITICAL
+
+
+def test_stderr_appended_when_both_present(tmp_path):
+    script = tmp_path / "check_both.sh"
+    script.write_text("#!/bin/sh\necho 'OK - all good'\necho 'extra detail' >&2\nexit 0\n")
+    script.chmod(script.stat().st_mode | stat.S_IEXEC)
+
+    config = {"commands": [{"name": "t", "command": str(script)}], "timeout": 5}
+    plugin = NagiosRunnerPlugin(config=config)
+    asyncio.run(plugin.initialize())
+    data = asyncio.run(plugin._collect_metrics())
+
+    assert "OK - all good" in data["t_output"]
+    assert "extra detail" in data["t_output"]
+    assert data["t_status_code"] == NAGIOS_OK
+
+
+def test_negative_returncode_maps_to_unknown():
+    # kill -9 $$ kills the shell itself; asyncio sees returncode -9
+    config = {"commands": [{"name": "t", "command": "kill -9 $$"}], "timeout": 5}
+    plugin = NagiosRunnerPlugin(config=config)
+    asyncio.run(plugin.initialize())
+    data = asyncio.run(plugin._collect_metrics())
+
+    assert data["t_status_code"] == NAGIOS_UNKNOWN
+    assert "signal" in data["t_output"].lower()