Add date range picker theme integration test

Introduces a new integration test for the DateRangePicker theme on macOS, including a golden screenshot for visual regression. This ensures custom theme properties are correctly applied and rendered.

Author: InesaFitsner

sdk/python/packages/flet/integration_tests/controls/golden/macos/date_range_picker_theme/theme1.png

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+import datetime
+
+import pytest
+import pytest_asyncio
+
+import flet as ft
+import flet.testing as ftt
+
+
+# Create a new flet_app instance for each test method
+@pytest_asyncio.fixture(scope="function", autouse=True)
+def flet_app(flet_app_function):
+    return flet_app_function
+
+
+@pytest.mark.asyncio(loop_scope="function")
+async def test_date_picker_theme(flet_app: ftt.FletTestApp, request):
+    flet_app.page.theme = ft.Theme(
+        date_picker_theme=ft.DatePickerTheme(
+            bgcolor=ft.Colors.GREEN_200,
+            range_picker_bgcolor=ft.Colors.GREEN_100,
+            range_picker_elevation=10,
+            range_picker_header_foreground_color=ft.Colors.GREEN_900,
+            range_picker_header_headline_text_style=ft.TextStyle(italic=True),
+            range_picker_shape=ft.BeveledRectangleBorder(
+                radius=20,
+                side=ft.BorderSide(5, color=ft.Colors.PURPLE),
+            ),
+            range_picker_header_help_text_style=ft.TextStyle(color=ft.Colors.GREEN_800),
+            range_selection_bgcolor=ft.Colors.YELLOW_200,
+            range_selection_overlay_color=ft.Colors.YELLOW_400,
+        )
+    )
+
+    dp = ft.DateRangePicker(
+        current_date=datetime.datetime(year=2025, month=8, day=15),
+        first_date=datetime.datetime(year=2000, month=10, day=1),
+        last_date=datetime.datetime(year=2025, month=10, day=1),
+        start_value=datetime.datetime(year=2000, month=10, day=7),
+        end_value=datetime.datetime(year=2000, month=10, day=15),
+    )
+    flet_app.page.enable_screenshots = True
+    flet_app.page.window.width = 400
+    flet_app.page.window.height = 600
+    flet_app.page.show_dialog(dp)
+    flet_app.page.update()
+    await flet_app.tester.pump_and_settle()
+
+    flet_app.assert_screenshot(
+        "theme1",
+        await flet_app.page.take_screenshot(
+            pixel_ratio=flet_app.screenshots_pixel_ratio
+        ),
+    )