Fix label immutability help text (#37748)

- [x] Input data is properly validated, `SELECT *` is avoided, SQL
injection is prevented (using placeholders for values in statements)

## Testing

- [x] Added/updated automated tests
- [x] QA'd all new/changed functionality manually
- [x] Confirmed that the fix is not expected to adversely impact load
test results

---------

Co-authored-by: RachelElysia <71795832+RachelElysia@users.noreply.github.com>

frontend/pages/labels/components/LabelForm/LabelForm.tests.tsx

@@ -66,4 +66,77 @@ describe("LabelForm", () => {
       true
     );
   });
+
+  it("should not render immutable help text when no immutable fields are provided (ManualLabelForm without team)", () => {
+    render(
+      <LabelForm
+        onSave={noop}
+        onCancel={noop}
+        teamName={null}
+        immutableFields={[]}
+      />
+    );
+
+    // Help text container should not be in the document
+    expect(
+      screen.queryByText(
+        /are immutable\. To make changes, delete this label and create a new one\./
+      )
+    ).not.toBeInTheDocument();
+  });
+
+  it("should render correct immutable help text for a single field (ManualLabelForm with team)", () => {
+    render(
+      <LabelForm
+        onSave={noop}
+        onCancel={noop}
+        teamName={"Example Team"}
+        immutableFields={["teams"]}
+      />
+    );
+
+    expect(
+      screen.getByText(
+        "Label teams are immutable. To make changes, delete this label and create a new one."
+      )
+    ).toBeInTheDocument();
+  });
+
+  it("should render correct immutable help text for two fields (DynamicLabelForm without team)", () => {
+    const immutableFields = ["queries", "platforms"];
+
+    render(
+      <LabelForm
+        onSave={noop}
+        onCancel={noop}
+        teamName={null}
+        immutableFields={immutableFields}
+      />
+    );
+
+    expect(
+      screen.getByText(
+        "Label queries and platforms are immutable. To make changes, delete this label and create a new one."
+      )
+    ).toBeInTheDocument();
+
+    expect(immutableFields.length).toBe(2);
+  });
+
+  it("should render correct immutable help text for three fields (DynamicLabelForm with team)", () => {
+    render(
+      <LabelForm
+        onSave={noop}
+        onCancel={noop}
+        teamName={"Example Team"}
+        immutableFields={["teams", "queries", "platforms"]}
+      />
+    );
+
+    expect(
+      screen.getByText(
+        "Label teams, queries, and platforms are immutable. To make changes, delete this label and create a new one."
+      )
+    ).toBeInTheDocument();
+  });
 });

frontend/pages/labels/components/LabelForm/LabelForm.tsx

@@ -33,11 +33,19 @@ const generateDescriptionHelpText = (immutableFields: string[]) => {
   const SUFFIX =
     "are immutable. To make changes, delete this label and create a new one.";
 
-  return immutableFields.length === 1
-    ? `Label ${immutableFields[0]} ${SUFFIX}`
-    : `Label ${immutableFields
-        .slice(0, -1)
-        .join(", ")} and ${immutableFields.pop()} ${SUFFIX}`;
+  if (immutableFields.length === 1) {
+    return `Label ${immutableFields[0]} ${SUFFIX}`;
+  }
+
+  if (immutableFields.length === 2) {
+    // No comma for two items: "queries and platforms"
+    return `Label ${immutableFields[0]} and ${immutableFields[1]} ${SUFFIX}`;
+  }
+
+  // 3+ items: Oxford comma before "and"
+  const allButLast = immutableFields.slice(0, -1).join(", ");
+  const last = immutableFields.slice(-1);
+  return `Label ${allButLast}, and ${last} ${SUFFIX}`;
 };
 
 const LabelForm = ({