fix: add setup cell that has been deleted (#6916)

## 📝 Summary

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#123).
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Updated the condition for checking the existence of the setup cell to
use `state.cellIds` instead of `state.cellData`.
When deleteCell is called, it removes from cellIds, not cellData

## 🔍 Description of Changes

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## 📋 Checklist

- [x] I have read the [contributor
guidelines](https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md).
- [ ] For large changes, or changes that affect the public API: this
change was discussed or approved through an issue, on
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- [x] I have added tests for the changes made.
- [x] I have run the code and verified that it works as expected.

Author: Light2Dark

frontend/src/core/cells/__tests__/cells.test.ts

@@ -2109,7 +2109,7 @@ describe("cell reducer", () => {
     actions.addSetupCellIfDoesntExist({ code: "# Updated setup code" });
 
     // Check that the same setup cell was updated, not duplicated
-    expect(state.cellData[SETUP_CELL_ID].code).toBe("# Setup code");
+    expect(state.cellData[SETUP_CELL_ID].code).toBe("# Updated setup code");
     expect(state.cellData[SETUP_CELL_ID].edited).toBe(true);
     expect(state.cellIds.inOrderIds).toContain(SETUP_CELL_ID);
   });
@@ -2143,6 +2143,24 @@ describe("cell reducer", () => {
     expect(state.cellIds.inOrderIds).toContain(SETUP_CELL_ID);
   });
 
+  it("can delete and then create a new setup cell", () => {
+    // Create the setup cell
+    actions.addSetupCellIfDoesntExist({ code: "# Setup code" });
+
+    // Delete the setup cell
+    actions.deleteCell({ cellId: SETUP_CELL_ID });
+
+    // Create a new setup cell
+    actions.addSetupCellIfDoesntExist({ code: "# New code" });
+
+    // Check that the new setup cell was created
+    expect(state.cellData[SETUP_CELL_ID].id).toBe(SETUP_CELL_ID);
+    expect(state.cellData[SETUP_CELL_ID].name).toBe("setup");
+    expect(state.cellData[SETUP_CELL_ID].code).toBe("# New code");
+    expect(state.cellData[SETUP_CELL_ID].edited).toBe(true);
+    expect(state.cellIds.inOrderIds).toContain(SETUP_CELL_ID);
+  });
+
   it("can clear all outputs", () => {
     // Add a cell and give it output
     actions.createNewCell({

frontend/src/core/cells/cells.ts

@@ -1336,7 +1336,7 @@ const {
     }
 
     // First check if setup cell already exists
-    if (SETUP_CELL_ID in state.cellData) {
+    if (SETUP_CELL_ID in state.cellIds) {
       // Just focus on the existing setup cell
       return {
         ...state,