Fix AbortEvent tests to use valid AbortReason enum values after codegen update

The codegen regeneration changed AbortEvent.data.reason from a free-form
String to a strict AbortReason enum. Updated tests to use valid values
(user_initiated, user_abort) instead of the old strings.

Co-authored-by: edburns <75821+edburns@users.noreply.github.com>

Author: Copilot

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

@@ -736,7 +736,7 @@ void testParseAbortEvent() throws Exception {
                 {
                     "type": "abort",
                     "data": {
-                        "reason": "user_requested"
+                        "reason": "user_initiated"
                     }
                 }
                 """;
@@ -1987,14 +1987,14 @@ void testAbortEventAllFields() throws Exception {
                 {
                     "type": "abort",
                     "data": {
-                        "reason": "user_cancelled"
+                        "reason": "user_abort"
                     }
                 }
                 """;
 
         var event = (AbortEvent) parseJson(json);
         assertNotNull(event);
-        assertEquals("user_cancelled", event.getData().reason());
+        assertEquals(AbortReason.USER_ABORT, event.getData().reason());
     }
 
     @Test