fix runtime exceptions surfaced when the editor opens

Now that the editor split actually opens (T-197), it exposed latent
issues, plus a coincidental Claude-session crash in the same log:

- T-203: the _EditorDragHandle's slider Semantics had value +
  onIncrease/onDecrease but no increased/decreasedValue, so Flutter
  asserted on every semantics flush — add them. And opening the split
  reparented the Claude pane (direct child → Column/Expanded), tearing
  down its SelectableRegion mid selection-update ('selectable not in
  this registrar' / 'inactive element'); a stable GlobalKey on the
  workspace primary makes Flutter move the element instead.
- T-202: rate_limit_event.resetsAt arrives as a unix-epoch number but
  was cast `as String?`, throwing in the stream-json line parser. Accept
  a num (epoch) or an ISO string.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/builtin/claude/stream_json_session_test.dart

@@ -271,6 +271,18 @@ void main() {
       expect(statuses.last.rateLimitInfo, contains('rate limited'));
       expect(statuses.last.rateLimitInfo, contains('resets'));
     });
+
+    test('rate_limit_event with a numeric (epoch) resetsAt does not crash', () async {
+      // Claude sends resetsAt as a unix-epoch number, not a string — the
+      // old `as String?` cast threw 'int is not a subtype of String?'.
+      proc.emit(jsonEncode({
+        'type': 'rate_limit_event',
+        'rate_limit_info': {'status': 'rate_limited', 'resetsAt': 1780000000},
+      }));
+      await Future<void>.delayed(Duration.zero);
+      expect(statuses.last.rateLimitInfo, contains('rate limited'));
+      expect(statuses.last.rateLimitInfo, contains('resets'));
+    });
   });
 
   group('token streaming via stream_event (T-168, shape verified by T-184)', () {