Use semicolon separator for pgBackRest error messages

Concatenate multiple ERROR/WARN lines with "; " instead of " " for
improved readability when debugging backup issues.
Updated unit test to match new separator format.
Cherry-picked from #1328 (commit 1f1b601).

Signed-off-by: Marcelo Henrique Neppel <marcelo.neppel@canonical.com>

Author: marceloneppel

src/backups.py

@@ -383,7 +383,7 @@ def _extract_error_message(stdout: str, stderr: str) -> str:
 
         # If we found error/warning lines, return them joined
         if error_lines:
-            return " ".join(error_lines)
+            return "; ".join(error_lines)
 
         # Otherwise return the last non-empty line from stderr or stdout
         if stderr.strip():

tests/unit/test_backups.py

@@ -77,7 +77,7 @@ def test_extract_error_message_with_multiple_errors():
 P00 ERROR: second error"""
     stderr = ""
     result = PostgreSQLBackups._extract_error_message(stdout, stderr)
-    assert result == "ERROR: first error WARN: warning message ERROR: second error"
+    assert result == "ERROR: first error; WARN: warning message; ERROR: second error"
 
 
 def test_extract_error_message_with_empty_output():