[gitlab-org/gitlab] discussions store: updateNote uses deep merge where splice used full replacement

### ROOT CAUSE The issue arises because the `UPDATE_NOTE` mutation uses `lodash.merge`, which performs a deep merge, preserving existing properties that might no longer be present in the server's response. This can lead to stale data if the API response structure changes. The original `splice` method replaced the entire note object, correctly removing missing properties but breaking reactivity. ### CODE FIX To address the issue while maintaining reactivity and ensuring that missing properties are removed, we'll switch to using `Object.assign` for top-level properties. This approach updates the existing object's properties without replacing the entire object, preserving reactivity. Additionally, we'll ensure that any properties not present in the new data are removed. **Step-by-Step Explanation:** 1. **Use `Object.assign`:** Replace `lodash.merge` with `Object.assign` to update the existing note's properties at the top level. This maintains the same object reference, preserving reactivity. 2. **Remove Missing Properties:** After updating, iterate over the existing properties and delete any that are not present in the new data to ensure missing properties are removed. **Modified Code:**

// In mutations.js, replace the updateNote method
export const updateNote = (state, { note, noteId }) => {
  const existingNote = state.notes[noteId];
  // Update existingNote with new data using Object.assign
  Object.assign(existingNote, note);
  // Remove any properties not present in the new note data
  const keysToKeep = Object.keys(note);
  Object.keys(existingNote).forEach(key => {
    if (!keysToKeep.includes(key)) {
      delete existingNote[key];
    }
  });
};


This solution ensures that the note's properties are updated correctly, reactivity is maintained, and any missing properties are removed, addressing the root cause effectively.