Fix Java PingResponse timestamp deserialization for ISO-8601 format

The Copilot CLI now returns ISO-8601 timestamp strings instead of numeric
epoch milliseconds. Update PingResponse.timestamp from long to String and
PingResult.timestamp from Long to String. Update corresponding test
assertions accordingly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

java/src/generated/java/com/github/copilot/sdk/generated/rpc/PingResult.java

@@ -23,8 +23,8 @@
 public record PingResult(
     /** Echoed message (or default greeting) */
     @JsonProperty("message") String message,
-    /** Server timestamp in milliseconds */
-    @JsonProperty("timestamp") Long timestamp,
+    /** Server timestamp as an ISO-8601 string */
+    @JsonProperty("timestamp") String timestamp,
     /** Server protocol version number */
     @JsonProperty("protocolVersion") Long protocolVersion
 ) {

java/src/main/java/com/github/copilot/sdk/json/PingResponse.java

@@ -20,8 +20,8 @@
 public record PingResponse(
         /** The echo message from the server. */
         @JsonProperty("message") String message,
-        /** The server timestamp in milliseconds since epoch. */
-        @JsonProperty("timestamp") long timestamp,
+        /** The server timestamp as an ISO-8601 string. */
+        @JsonProperty("timestamp") String timestamp,
         /**
          * The SDK protocol version supported by the server. The SDK validates that this
          * version matches the expected version to ensure compatibility.

java/src/test/java/com/github/copilot/sdk/CopilotClientTest.java

@@ -92,7 +92,8 @@ void testStartAndConnectUsingStdio() throws Exception {
 
             PingResponse pong = client.ping("test message").get();
             assertEquals("pong: test message", pong.message());
-            assertTrue(pong.timestamp() >= 0);
+            assertNotNull(pong.timestamp());
+            assertFalse(pong.timestamp().isEmpty());
 
             client.stop().get();
             assertEquals(ConnectionState.DISCONNECTED, client.getState());

java/src/test/java/com/github/copilot/sdk/generated/rpc/GeneratedRpcRecordsCoverageTest.java

@@ -32,9 +32,9 @@ void pingParams_record() {
 
     @Test
     void pingResult_record() {
-        var result = new PingResult("pong", 1234L, 2L);
+        var result = new PingResult("pong", "2026-01-01T00:00:00Z", 2L);
         assertEquals("pong", result.message());
-        assertEquals(1234L, result.timestamp());
+        assertEquals("2026-01-01T00:00:00Z", result.timestamp());
         assertEquals(2L, result.protocolVersion());
     }
 
@@ -471,9 +471,9 @@ void sessionWorkspaceReadFileParams_record() {
 
     @Test
     void pingResult_fields() {
-        var result = new PingResult("pong", 9999L, 1L);
+        var result = new PingResult("pong", "2026-06-15T10:30:00Z", 1L);
         assertEquals("pong", result.message());
-        assertEquals(9999L, result.timestamp());
+        assertEquals("2026-06-15T10:30:00Z", result.timestamp());
         assertEquals(1L, result.protocolVersion());
     }