feat: split client_helpers into domain-specific modules

Splits the monolithic client_helpers.py into routing.py, client_cert.py, config_helpers.py, and method_helpers.py to improve readability and maintainability.
Updates gapic_v1 exports and test imports accordingly.

Author: hebaalazzeh

packages/google-api-core/google/api_core/gapic_v1/__init__.py

@@ -12,20 +12,26 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from google.api_core.gapic_v1 import client_helpers
+from google.api_core.gapic_v1 import client_cert
 from google.api_core.gapic_v1 import client_info
 from google.api_core.gapic_v1 import config
 from google.api_core.gapic_v1 import config_async
+from google.api_core.gapic_v1 import config_helpers
 from google.api_core.gapic_v1 import method
 from google.api_core.gapic_v1 import method_async
+from google.api_core.gapic_v1 import method_helpers
+from google.api_core.gapic_v1 import routing
 from google.api_core.gapic_v1 import routing_header
 
 __all__ = [
-    "client_helpers",
+    "client_cert",
     "client_info",
     "config",
     "config_async",
+    "config_helpers",
     "method",
     "method_async",
+    "method_helpers",
+    "routing",
     "routing_header",
 ]

packages/google-api-core/google/api_core/gapic_v1/client_cert.py

@@ -0,0 +1,84 @@
+# -*- coding: utf-8 -*-
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+"""Helpers for client certificate handling and mTLS authentication."""
+
+import os
+from typing import Any, Optional
+
+from google.auth.transport import mtls  # type: ignore
+
+
+def use_client_cert_effective() -> bool:
+    """Returns whether client certificate should be used for mTLS if the
+    google-auth version supports should_use_client_cert automatic mTLS
+    enablement.
+
+    Alternatively, read from the GOOGLE_API_USE_CLIENT_CERTIFICATE env var.
+
+    Returns:
+        bool: whether client certificate should be used for mTLS
+    Raises:
+        ValueError: (If using a version of google-auth without
+        should_use_client_cert and GOOGLE_API_USE_CLIENT_CERTIFICATE is
+        set to an unexpected value.)
+    """
+    # check if google-auth version supports should_use_client_cert for
+    # automatic mTLS enablement
+    if hasattr(mtls, "should_use_client_cert"):  # pragma: NO COVER
+        return mtls.should_use_client_cert()
+    else:  # pragma: NO COVER
+        # if unsupported, fallback to reading from env var
+        use_client_cert_str = os.getenv(
+            "GOOGLE_API_USE_CLIENT_CERTIFICATE", "false"
+        ).lower()
+        if use_client_cert_str not in ("true", "false"):
+            raise ValueError(
+                "Environment variable `GOOGLE_API_USE_CLIENT_CERTIFICATE` "
+                "must be either `true` or `false`"
+            )
+        return use_client_cert_str == "true"
+
+
+def get_client_cert_source(
+    provided_cert_source: Optional[Any], use_cert_flag: bool
+) -> Optional[Any]:
+    """Return the client cert source to be used by the client.
+
+    Args:
+        provided_cert_source (bytes): The client certificate source provided.
+        use_cert_flag (bool): A flag indicating whether to use the
+            client certificate.
+
+    Returns:
+        bytes or None: The client cert source to be used by the client.
+    """
+    client_cert_source = None
+    if use_cert_flag:
+        if provided_cert_source:
+            client_cert_source = provided_cert_source
+        elif (
+            hasattr(mtls, "has_default_client_cert_source")
+            and mtls.has_default_client_cert_source()
+        ):
+            client_cert_source = mtls.default_client_cert_source()
+    return client_cert_source
+
+
+# Backward compatibility aliases for private methods
+# Previously, gapic-generator-python generated clients used these methods
+_use_client_cert_effective = use_client_cert_effective
+_get_client_cert_source = get_client_cert_source