[PSM Interop] Don't initialize the secondary context when not needed (#34130)

Fixes an issue when an active context selected automatically picked up
as context for `secondary_k8s_api_manager`.

This was introducing an error in GAMMA Baseline PoC

```
sys:1: ResourceWarning: unclosed <ssl.SSLSocket fd=4, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=0, laddr=('100.71.2.143', 56723), raddr=('35.199.174.232', 443)>
```

Here's how the secondary context is incorrectly falls back to the
default context when `--secondary_kube_context` is not set:
```
k8s.py:142] Using kubernetes context "gke_grpc-testing_us-central1-a_psm-interop-security", active host: https://35.202.85.90
k8s.py:142] Using kubernetes context "None", active host: https://35.202.85.90
```

tools/run_tests/xds_k8s_test_driver/framework/xds_k8s_flags.py

@@ -68,3 +68,27 @@ flags.mark_flags_as_required(
         "client_image",
     ]
 )
+
+
+def require_secondary_context(filename: str):
+    """
+    Makes secondary_kube_context flag required and adds the non-empty validator.
+
+    Typical usage example:
+
+      xds_k8s_flags.require_secondary_context(__file__)
+    """
+    flags.mark_flag_as_required("secondary_kube_context")
+
+    def _val_not_empty(val: str) -> bool:
+        # Do not allow whitespace-only values to produce a better error.
+        return bool(val.strip())
+
+    flags.register_validator(
+        "secondary_kube_context",
+        _val_not_empty,
+        message=(
+            f"{filename} requires non-empty secondary_kube_context to access"
+            " the secondary k8s cluster"
+        ),
+    )

tools/run_tests/xds_k8s_test_driver/framework/xds_k8s_testcase.py

@@ -118,7 +118,7 @@ class XdsKubernetesBaseTestCase(base_testcase.BaseTestCase):
     gcp_api_manager: gcp.api.GcpApiManager
     gcp_service_account: Optional[str]
     k8s_api_manager: k8s.KubernetesApiManager
-    secondary_k8s_api_manager: k8s.KubernetesApiManager
+    secondary_k8s_api_manager: Optional[k8s.KubernetesApiManager] = None
     network: str
     project: str
     resource_prefix: str
@@ -205,9 +205,10 @@ class XdsKubernetesBaseTestCase(base_testcase.BaseTestCase):
         cls.k8s_api_manager = k8s.KubernetesApiManager(
             xds_k8s_flags.KUBE_CONTEXT.value
         )
-        cls.secondary_k8s_api_manager = k8s.KubernetesApiManager(
-            xds_k8s_flags.SECONDARY_KUBE_CONTEXT.value
-        )
+        if xds_k8s_flags.SECONDARY_KUBE_CONTEXT.value is not None:
+            cls.secondary_k8s_api_manager = k8s.KubernetesApiManager(
+                xds_k8s_flags.SECONDARY_KUBE_CONTEXT.value
+            )
         cls.gcp_api_manager = gcp.api.GcpApiManager()
 
         # Other
@@ -236,7 +237,8 @@ class XdsKubernetesBaseTestCase(base_testcase.BaseTestCase):
     @classmethod
     def tearDownClass(cls):
         cls.k8s_api_manager.close()
-        cls.secondary_k8s_api_manager.close()
+        if cls.secondary_k8s_api_manager is not None:
+            cls.secondary_k8s_api_manager.close()
         cls.gcp_api_manager.close()
 
     def setUp(self):

tools/run_tests/xds_k8s_test_driver/tests/failover_test.py

@@ -25,6 +25,7 @@ from framework.test_app.runners.k8s import k8s_xds_server_runner
 
 logger = logging.getLogger(__name__)
 flags.adopt_module_key_flags(xds_k8s_testcase)
+xds_k8s_flags.require_secondary_context(__file__)
 
 # Type aliases
 _Lang = skips.Lang