add legacy to new format timestamp test

Author: vpodlesnyak

firebase-firestore/src/commonTest/kotlin/dev/gitlive/firebase/firestore/firestore.kt

@@ -459,6 +459,31 @@ class FirebaseFirestoreTest {
         assertNotEquals(DoubleAsTimestampSerializer.serverTimestamp, pendingWritesSnapshot.data(DoubleTimestamp.serializer(), ServerTimestampBehavior.ESTIMATE).time)
     }
 
+    @Test
+    fun testLegacyDoubleTimestampWriteNewFormatRead() = runTest {
+        @Serializable
+        data class LegacyDocument(
+            @Serializable(with = DoubleAsTimestampSerializer::class)
+            val time: Double
+        )
+
+        @Serializable
+        data class NewDocument(
+            val time: Timestamp
+        )
+
+        val doc = Firebase.firestore
+            .collection("testLegacyDoubleTimestampEncodeDecode")
+            .document("testLegacy")
+
+        val ms = 12345678.0
+
+        doc.set(LegacyDocument(time = ms))
+
+        val fetched: NewDocument = doc.get().data()
+        assertEquals(ms, fetched.time.toMilliseconds())
+    }
+
     @Test
     fun testQueryByTimestamp() = runTest {
         @Serializable