fix: preserve shell exit code with stderr output

Author: VectorPeak

src/google/adk/tools/skill_toolset.py

@@ -564,8 +564,13 @@ async def execute_script_async(
             stdout = parsed.get("stdout", "")
             stderr = parsed.get("stderr", "")
             rc = parsed.get("returncode", 0)
-            if rc != 0 and not stderr:
-              stderr = f"Exit code {rc}"
+            if rc != 0:
+              exit_code_message = f"Exit code {rc}"
+              stderr = (
+                  f"{stderr.rstrip()}\n{exit_code_message}"
+                  if stderr
+                  else exit_code_message
+              )
         except (json.JSONDecodeError, ValueError):
           pass
 
@@ -745,6 +750,7 @@ def _build_wrapper_code(
           "        _r = subprocess.run(",
           f"          {arr!r},",
           "          capture_output=True, text=True,",
+          # Keep shell output decoding independent from the host locale.
           "          encoding='utf-8', errors='replace',",
           f"          timeout={timeout!r}, cwd=td,",
           "        )",