[terminal buffering] Add support for the "mode 2026"

That task is definitely way more complicated that it seemed to be 😅

tests/utilities/test_app.py

@@ -13,8 +13,10 @@ from rich.console import Console
 
 from textual import events, errors
 from textual._clock import _Clock
-from textual.app import App, ComposeResult, WINDOWS
+from textual.app import WINDOWS
 from textual._context import active_app
+from textual._terminal_features import TerminalSupportedFeatures
+from textual.app import App, ComposeResult
 from textual.driver import Driver
 from textual.geometry import Size, Region
 
@@ -45,9 +47,13 @@ class AppTest(App):
         # Let's disable all features by default
         self.features = frozenset()
 
-        # We need this so the `CLEAR_SCREEN_SEQUENCE` is always sent for a screen refresh,
+        # We need this so the iTerm2 `CLEAR_SCREEN_SEQUENCE` is always sent for a screen refresh,
         # whatever the environment:
-        self._sync_available = True
+        # (we use it to slice the output into distinct full screens displays)
+        self._terminal_features = TerminalSupportedFeatures(
+            iterm2_synchronized_update=True,
+            mode2026_synchronized_update=False,
+        )
 
         self._size = size
         self._console = ConsoleTest(width=size.width, height=size.height)