fix(terminal): send SGR mouse wheel reports with the button codes app… (#15817)

* fix(terminal): send SGR mouse wheel reports with the button codes apps expect

xterm.dart 4.0.0 encodes the wheel buttons as 64+4..64+7 rather than
64+0..64+3, so the low bits land on the modifier field and every wheel
report the terminal emits reads as wheel-with-Shift. Strict full-screen
applications reject the modified event, which is why neither the mouse
wheel nor the trackpad scrolls anything once the peer application takes
over the alternate screen.

Install a mouse handler that keeps every upstream reporting decision and
only re-encodes the wheel buttons as 64..67. Non-wheel reports pass
through untouched, and the emitted bytes stay identical once upstream
ships the same fix, so this can be dropped without a behavior change.

Upstream: TerminalStudio/xterm.dart#238

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(terminal): correct the wheel report row, drop the wasted report build

Address review feedback on the wheel button fix:

- The X10/utf row was encoded as `32 + y + 1` while y is already 1-based, so
  every normal-mode report pointed one row too low and the `y > limit` guard
  disagreed with what it emitted.
- Gate the wheel path on `mouseMode.reportScroll` and the button state instead
  of building and discarding a full report string from `defaultMouseHandler`
  on every scroll tick. This also makes the hardcoded SGR 'M' provably right,
  since a wheel release now returns before the report is built.
- Derive the wire code as `id - 4` and drop `_wheelButtonId`, whose `default`
  branch was unreachable and defeated enum exhaustiveness.
- Assign `mouseHandler` after construction so the `Terminal(...)` line stays
  untouched.

Cover the utf, urxvt, null-byte overflow and click-only branches, and assert
that TerminalModel actually installs the handler.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

flutter/test/terminal_model_lifecycle_test.dart

@@ -3,6 +3,7 @@ import 'dart:async';
 import 'package:flutter_hbb/models/model.dart';
 import 'package:flutter_hbb/models/terminal_model.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:xterm/xterm.dart';
 
 class _FakeFFI implements FFI {
   @override
@@ -48,4 +49,20 @@ void main() {
     expect(clearedCtrlLock, isFalse);
     expect(model.debugBufferedInputCount, 0);
   });
+
+  test('builds its terminal with the wheel button fix', () {
+    final model = TerminalModel(_FakeFFI());
+    addTearDown(model.dispose);
+
+    final captured = <String>[];
+    model.terminal.onOutput = captured.add;
+    model.terminal.write('\x1b[?1000h\x1b[?1006h');
+    model.terminal.mouseInput(
+      TerminalMouseButton.wheelUp,
+      TerminalMouseButtonState.down,
+      const CellOffset(10, 5),
+    );
+
+    expect(captured.single, '\x1b[<64;11;6M');
+  });
 }