xDS interop: Handle the edge case when rand deployment_id is all nums (#30901)

Fun edge case: when `rand_string()` happen to generate numbers only,
yaml interprets `deployment_id` label value as an integer,
but k8s expects label values to be strings.

K8s responds with a barely readable 400 Bad Request error:
`ReadString: expects \" or n, but found 9, error found in #10 byte of ...|ent_id`.

Prepending deployment name forces deployment_id into a string,
as well as it's just a better description.

tools/run_tests/xds_k8s_test_driver/framework/test_app/runners/k8s/k8s_base_runner.py

@@ -225,12 +225,28 @@ class KubernetesBaseRunner(base_runner.BaseRunner):
         return resource
 
     def _create_deployment(self, template, **kwargs) -> k8s.V1Deployment:
+        # Not making deployment_name an explicit kwarg to be consistent with
+        # the rest of the _create_* methods, which pass kwargs as-is
+        # to _create_from_template(), so that the kwargs dict is unpacked into
+        # template variables and their values.
+        if 'deployment_name' not in kwargs:
+            raise TypeError('Missing required keyword-only argument: '
+                            'deployment_name')
+
         # Automatically apply random deployment_id to use in the matchLabels
         # to prevent selecting pods in the same namespace belonging to
         # a different deployment.
         if 'deployment_id' not in kwargs:
-            kwargs['deployment_id'] = framework.helpers.rand.rand_string(
-                lowercase=True)
+            rand_id: str = framework.helpers.rand.rand_string(lowercase=True)
+            # Fun edge case: when rand_string() happen to generate numbers only,
+            # yaml interprets deployment_id label value as an integer,
+            # but k8s expects label values to be strings. Lol. K8s responds
+            # with a barely readable 400 Bad Request error: 'ReadString: expects
+            # \" or n, but found 9, error found in #10 byte of ...|ent_id'.
+            # Prepending deployment name forces deployment_id into a string,
+            # as well as it's just a better description.
+            kwargs['deployment_id'] = f'{kwargs["deployment_name"]}-{rand_id}'
+
         deployment = self._create_from_template(template, **kwargs)
         if not isinstance(deployment, k8s.V1Deployment):
             raise _RunnerError('Expected V1Deployment to be created '