Fixed card tests for control images; moved core controls to core folder (#5693)

* Move integration tests from material to core and update image docs

Integration test files and golden images for column, grid view, icon, image, and text controls were moved from the 'material' to the 'core' directory. Documentation and example image paths were updated accordingly. The image control example now uses a base64-encoded image instead of a remote URL, and the corresponding test was updated to match.

* Add similarity_threshold to screenshot assertions

Introduces a similarity_threshold parameter to screenshot assertion methods in tests and FletTestApp, allowing configurable image comparison sensitivity for integration tests.

Author: InesaFitsner

sdk/python/packages/flet/docs/controls/column.md

@@ -1,7 +1,7 @@
 ---
 class_name: flet.Column
 examples: ../../examples/controls/column
-example_images: ../test-images/examples/material/golden/macos/column
+example_images: ../test-images/examples/core/golden/macos/column
 example_media: ../examples/controls/column/media
 ---
 

sdk/python/packages/flet/docs/controls/gridview.md

@@ -1,7 +1,7 @@
 ---
 class_name: flet.GridView
 examples: ../../examples/controls/grid_view
-example_images: ../test-images/examples/material/golden/macos/grid_view
+example_images: ../test-images/examples/core/golden/macos/grid_view
 example_media: ../examples/controls/grid_view/media
 ---
 

sdk/python/packages/flet/docs/controls/icon.md

@@ -1,7 +1,7 @@
 ---
 class_name: flet.Icon
 examples: ../../examples/controls/icon
-example_images: ../test-images/examples/material/golden/macos/icon
+example_images: ../test-images/examples/core/golden/macos/icon
 example_media: ../examples/controls/icon/media
 ---
 

sdk/python/packages/flet/docs/controls/image.md

@@ -1,7 +1,7 @@
 ---
 class_name: flet.Image
 examples: ../../examples/controls/image
-example_images: ../test-images/examples/material/golden/macos/image
+example_images: ../test-images/examples/core/golden/macos/image
 example_media: ../examples/controls/image/media
 ---
 

sdk/python/packages/flet/docs/controls/text.md

@@ -1,7 +1,7 @@
 ---
 class_name: flet.Text
 examples: ../../examples/controls/text
-example_images: ../test-images/examples/material/golden/macos/text
+example_images: ../test-images/examples/core/golden/macos/text
 example_media: ../examples/controls/text/media
 ---