test(02-03): add failing secure auth state store contract

- Covers write overwrite semantics
- Covers clear removing stored AuthState JSON

composeApp/src/commonTest/kotlin/dev/ulfrx/recipe/auth/SecureAuthStateStoreContractTest.kt

@@ -0,0 +1,27 @@
+package dev.ulfrx.recipe.auth
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNull
+
+class SecureAuthStateStoreContractTest {
+    @Test
+    fun writeOverwritesPreviousValueAndReadReturnsLatest() {
+        val store = SecureAuthStateStore()
+
+        store.write("""{"refresh_token":"first"}""")
+        store.write("""{"refresh_token":"second"}""")
+
+        assertEquals("""{"refresh_token":"second"}""", store.read())
+    }
+
+    @Test
+    fun clearRemovesStoredValue() {
+        val store = SecureAuthStateStore()
+
+        store.write("""{"refresh_token":"stored"}""")
+        store.clear()
+
+        assertNull(store.read())
+    }
+}