Convert MerkleProof timestamp from Long to java.time.Instant

Co-authored-by: mitchelllisle <18128531+mitchelllisle@users.noreply.github.com>

Author: Copilot

src/main/scala/org/mitchelllisle/analysers/MerkleTree.scala

@@ -3,6 +3,7 @@ package org.mitchelllisle.analysers
 import org.apache.spark.sql.{DataFrame, functions => F}
 import org.apache.spark.sql.types._
 import java.security.MessageDigest
+import java.time.Instant
 import scala.annotation.tailrec
 
 /** MerkleTreeAnalyser provides cryptographic proof capabilities for data retention and deletion verification.
@@ -14,7 +15,7 @@ object MerkleTree {
                           rootHash: String,
                           recordCount: Long,
                           leafHashes: Seq[String],
-                          timestamp: Long = System.currentTimeMillis()
+                          timestamp: Instant = Instant.now()
                         )
 
   case class DeletionProof(
@@ -98,7 +99,7 @@ object MerkleTree {
       rootHash = rootHash,
       recordCount = recordCount,
       leafHashes = leafHashes,
-      timestamp = System.currentTimeMillis()
+      timestamp = Instant.now()
     )
   }
 

src/test/scala/MerkleTreeTest.scala

@@ -1,6 +1,7 @@
 import org.apache.spark.sql.DataFrame
 import org.mitchelllisle.analysers.MerkleTree
 import org.scalatest.flatspec.AnyFlatSpec
+import java.time.Instant
 
 class MerkleTreeTest extends AnyFlatSpec with SparkFunSuite {
   import spark.implicits._
@@ -21,7 +22,8 @@ class MerkleTreeTest extends AnyFlatSpec with SparkFunSuite {
     assert(proof.rootHash.nonEmpty)
     assert(proof.recordCount == 4)
     assert(proof.leafHashes.length == 4)
-    assert(proof.timestamp > 0)
+    assert(proof.timestamp.isInstanceOf[Instant])
+    assert(proof.timestamp.isBefore(Instant.now().plusSeconds(1)))
   }
 
   "apply" should "produce same result as createMerkleProof" in {
@@ -183,4 +185,14 @@ class MerkleTreeTest extends AnyFlatSpec with SparkFunSuite {
 
     assert(hashes1.sameElements(hashes2))
   }
+
+  "timestamp" should "use proper Instant type and be recent" in {
+    val beforeTime = Instant.now().minusSeconds(1)
+    val proof = MerkleTree.createMerkleProof(testData, columns, idColumn)
+    val afterTime = Instant.now().plusSeconds(1)
+
+    assert(proof.timestamp.isInstanceOf[Instant])
+    assert(proof.timestamp.isAfter(beforeTime))
+    assert(proof.timestamp.isBefore(afterTime))
+  }
 }