Fix 500 panic when generated Kubernetes namespace is too long (#6349)

# Description

Adding logic to CreateAppScopedNamespace to check if the generated name
is too long

## Type of change

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- This pull request fixes a bug in Radius and has an approved issue
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Fixes: #6292

## Auto-generated summary

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### <samp>🤖 Generated by Copilot at 71bf2ae</samp>

### Summary
🧪🔒🚀

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1. 🧪 - This emoji represents testing, and it can be used for the first
change that adds a unit test case for the validation check.
2. 🔒 - This emoji represents security or validation, and it can be used
for the second change that adds a validation check for the generated
namespace name.
3. 🚀 - This emoji represents deployment or launching, and it can be used
for the overall feature of creating application scoped namespaces.
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Add validation and testing for application scoped namespace names.
Ensure that the namespace name derived from the environment and
application names in `updatefilter.go` is valid for Kubernetes, and add
a unit test case in `updatefilter_test.go` to verify the behavior.

> _`CreateAppScopedNamespace`_
> _Validates namespace name - spring_
> _Bad request if too long_

### Walkthrough
* Validate namespace name for application scoped to environment
([link](https://github.com/radius-project/radius/pull/6349/files?diff=unified&w=0#diff-0ac671e5a672d80935544b559d7bed83e9456c8895f594b400bc86111e1ca9d6L71-R78))
* Test validation check for namespace name
([link](https://github.com/radius-project/radius/pull/6349/files?diff=unified&w=0#diff-9d9a183263ff9a4aecc0001864dfb4346a16375fdc7263c2e3295fa30cfb793dR162-R200))

pkg/corerp/frontend/controller/applications/updatefilter.go

@@ -68,7 +68,14 @@ func CreateAppScopedNamespace(ctx context.Context, newResource, oldResource *dat
 			return rest.NewBadRequestResponse(fmt.Sprintf("Environment %s could not be constructed: %s",
 				newResource.Properties.Environment, err.Error())), nil
 		}
-		kubeNamespace = kubernetes.NormalizeResourceName(fmt.Sprintf("%s-%s", envNamespace, serviceCtx.ResourceID.Name()))
+
+		namespace := fmt.Sprintf("%s-%s", envNamespace, serviceCtx.ResourceID.Name())
+		if !kubernetes.IsValidObjectName(namespace) {
+			return rest.NewBadRequestResponse(fmt.Sprintf("Application namespace '%s' could not be created: the combination of application and environment names is too long.",
+				namespace)), nil
+		}
+
+		kubeNamespace = kubernetes.NormalizeResourceName(namespace)
 	}
 
 	// Check if another environment resource is using namespace

pkg/corerp/frontend/controller/applications/updatefilter_test.go

@@ -159,6 +159,45 @@ func TestCreateAppScopedNamespace_invalid_property(t *testing.T) {
 		KubeClient:    k8sutil.NewFakeKubeClient(nil),
 	}
 
+	t.Run("generated namespace is invalid", func(t *testing.T) {
+		longAppID := "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/radius-test-rg/providers/applications.core/applications/this-is-a-very-long-application-name-that-is-invalid"
+		longEnvID := "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/radius-test-rg/providers/applications.core/environments/this-is-a-very-long-environment-name-that-is-invalid"
+
+		tCtx.MockSP.EXPECT().GetStorageClient(gomock.Any(), gomock.Any()).Return(tCtx.MockSC, nil).Times(1)
+
+		envdm := &datamodel.Environment{
+			Properties: datamodel.EnvironmentProperties{
+				Compute: rpv1.EnvironmentCompute{
+					Kind: rpv1.KubernetesComputeKind,
+				},
+			},
+		}
+
+		tCtx.MockSC.
+			EXPECT().
+			Get(gomock.Any(), gomock.Any()).
+			Return(rpctest.FakeStoreObject(envdm), nil)
+
+		newResource := &datamodel.Application{
+			Properties: datamodel.ApplicationProperties{
+				BasicResourceProperties: rpv1.BasicResourceProperties{
+					Environment: longEnvID,
+				},
+			},
+		}
+
+		id, err := resources.ParseResource(longAppID)
+		require.NoError(t, err)
+		armctx := &v1.ARMRequestContext{ResourceID: id}
+		ctx := v1.WithARMRequestContext(tCtx.Ctx, armctx)
+
+		resp, err := CreateAppScopedNamespace(ctx, newResource, nil, &opts)
+		require.NoError(t, err)
+		res := resp.(*rest.BadRequestResponse)
+
+		require.Equal(t, "Application namespace 'this-is-a-very-long-environment-name-that-is-invalid-this-is-a-very-long-application-name-that-is-invalid' could not be created: the combination of application and environment names is too long.", res.Body.Error.Message)
+	})
+
 	t.Run("invalid namespace", func(t *testing.T) {
 		tCtx.MockSC.EXPECT().
 			Query(gomock.Any(), gomock.Any()).