✅ test: add unit test for atomWithReset utility (#2753)

* ✅ test: add unit test for atomWithReset utility

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* apply review comments.

tests/vanilla/utils/atomWithReset.test.ts

@@ -0,0 +1,35 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { createStore } from 'jotai/vanilla'
+import { RESET, atomWithReset } from 'jotai/vanilla/utils'
+
+describe('atomWithReset', () => {
+  let initialValue: number
+  let testAtom: any
+
+  beforeEach(() => {
+    vi.clearAllMocks()
+    initialValue = 10
+    testAtom = atomWithReset(initialValue)
+  })
+
+  it('should reset to initial value using RESET', () => {
+    const store = createStore()
+    store.set(testAtom, 123)
+    store.set(testAtom, RESET)
+    expect(store.get(testAtom)).toBe(initialValue)
+  })
+
+  it('should update atom with a new value', () => {
+    const store = createStore()
+    store.set(testAtom, 123)
+    store.set(testAtom, 30)
+    expect(store.get(testAtom)).toBe(30)
+  })
+
+  it('should update atom using a function', () => {
+    const store = createStore()
+    store.set(testAtom, 123)
+    store.set(testAtom, (prev: number) => prev + 10)
+    expect(store.get(testAtom)).toBe(133)
+  })
+})