Update packages/google-api-core/google/api_core/universe.py

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: hebaalazzeh

packages/google-api-core/google/api_core/universe.py

@@ -138,10 +138,12 @@ def get_default_mtls_endpoint(api_endpoint: Optional[str]) -> Optional[str]:
     port = f":{parsed.port}" if parsed.port else ""
 
     lowered_host = host.lower()
-    if lowered_host.endswith(".sandbox.googleapis.com"):
-        new_host = host[:-23] + ".mtls.sandbox.googleapis.com"
-    elif lowered_host.endswith(".googleapis.com"):
-        new_host = host[:-15] + ".mtls.googleapis.com"
+    suffix_sandbox = ".sandbox.googleapis.com"
+    suffix_google = ".googleapis.com"
+    if lowered_host.endswith(suffix_sandbox):
+        new_host = host[:-len(suffix_sandbox)] + ".mtls.sandbox.googleapis.com"
+    elif lowered_host.endswith(suffix_google):
+        new_host = host[:-len(suffix_google)] + ".mtls.googleapis.com"
     else:
         return api_endpoint