test(ssr): add SSR and hydration tests for 'getServerSnapshot'

tests/ssr.test.tsx

@@ -0,0 +1,44 @@
+import { StrictMode } from 'react'
+import { renderToString } from 'react-dom/server'
+import { describe, expect, it } from 'vitest'
+import { proxy, useSnapshot } from 'valtio'
+
+describe('ssr', () => {
+  it('should return initial snapshot in SSR', () => {
+    const obj = proxy({ count: 0 })
+
+    const Counter = () => {
+      const snap = useSnapshot(obj)
+      return <div>count: {snap.count}</div>
+    }
+
+    const view = renderToString(
+      <StrictMode>
+        <Counter />
+      </StrictMode>,
+    )
+
+    expect(view).toContain('count:')
+    expect(view).toContain('0')
+  })
+
+  it('should return the latest snapshot after proxy state changes', () => {
+    const obj = proxy({ count: 0 })
+
+    obj.count = 5
+
+    const Counter = () => {
+      const snap = useSnapshot(obj)
+      return <div>count: {snap.count}</div>
+    }
+
+    const view = renderToString(
+      <StrictMode>
+        <Counter />
+      </StrictMode>,
+    )
+
+    expect(view).toContain('count:')
+    expect(view).toContain('5')
+  })
+})