test(router): Custom partition template API create using `time` tag value is rejected

This removes the double negative from the error message and adds
coverage at the router's gRPC API level for the rejection of the bad
TagValue value.

data_types/src/partition_template.rs

@@ -451,7 +451,7 @@ mod serialization {
 
                         if value.contains(TAG_VALUE_KEY_TIME) {
                             return Err(ValidationError::InvalidTagValue(format!(
-                                "{TAG_VALUE_KEY_TIME} cannot not be used"
+                                "{TAG_VALUE_KEY_TIME} cannot be used"
                             )));
                         }
                     }

router/tests/grpc.rs

@@ -981,6 +981,38 @@ async fn test_invalid_strftime_partition_template() {
     );
 }
 
+#[tokio::test]
+async fn test_invalid_tag_value_partition_template() {
+    // Initialise a TestContext without a namespace autocreation policy.
+    let ctx = TestContextBuilder::default().build().await;
+
+    // Explicitly create a namespace with a custom partition template.
+    let req = CreateNamespaceRequest {
+        name: "bananas_test".to_string(),
+        retention_period_ns: None,
+        partition_template: Some(PartitionTemplate {
+            parts: vec![TemplatePart {
+                part: Some(template_part::Part::TagValue("time".into())),
+            }],
+        }),
+        service_protection_limits: None,
+    };
+
+    // Check namespace creation returned an error
+    let got = ctx
+        .grpc_delegate()
+        .namespace_service()
+        .create_namespace(Request::new(req))
+        .await;
+
+    assert_error!(
+        got,
+        ref status
+            if status.code() == Code::InvalidArgument
+                && status.message() == "invalid tag value in partition template: time cannot be used"
+    );
+}
+
 #[tokio::test]
 async fn test_namespace_partition_template_implicit_table_creation() {
     // Initialise a TestContext without a namespace autocreation policy.