fix: fix mypy (#15496)

Towards #15104

This is needed to unblock
#15457 .

See follow up issue
googleapis/proto-plus-python#558 for the
`prerelease` presubmit failure.

See the following mypy failure for `grafeas`

```
2026-01-16T22:02:28.7158467Z grafeas/grafeas_v1/services/grafeas/client.py:362: error: Incompatible default for argument "transport" (default has type "None", argument has type "str | GrafeasTransport") 
```

Author: parthea

.librarian/generator-input/client-post-processing/unique-grafeas-client.yaml

@@ -1957,13 +1957,13 @@ replacements:
           def __init__(
               self,
               *,
-              transport: Union[str, GrafeasTransport] = None,
+              transport: Optional[Union[str, GrafeasTransport]] = None,
               credentials: Optional[ga_credentials.Credentials] = None,
           ) -> None:
               """Instantiate the grafeas client.
 
               Args:
-                  transport (Union[str, ~.GrafeasTransport]): The
+                  transport (Optional[Union[str, ~.GrafeasTransport]]): The
                       transport to use.
                   credentials (Optional[google.auth.credentials.Credentials]): The
                       authorization credentials to attach to requests. These

packages/grafeas/grafeas/grafeas_v1/services/grafeas/client.py

@@ -359,13 +359,13 @@ def parse_common_location_path(path: str) -> Dict[str, str]:
     def __init__(
         self,
         *,
-        transport: Union[str, GrafeasTransport] = None,
+        transport: Optional[Union[str, GrafeasTransport]] = None,
         credentials: Optional[ga_credentials.Credentials] = None,
     ) -> None:
         """Instantiate the grafeas client.
 
         Args:
-            transport (Union[str, ~.GrafeasTransport]): The
+            transport (Optional[Union[str, ~.GrafeasTransport]]): The
                 transport to use.
             credentials (Optional[google.auth.credentials.Credentials]): The
                 authorization credentials to attach to requests. These