Support ShapeDecoration in widget image precaching for tests

[remax21]

📝 Description

This pull request improves the image precaching mechanism in widget tests by adding support for ShapeDecoration.

Previously, only BoxDecoration was checked when attempting to extract and precache background images in decorated widgets. However, ShapeDecoration can also contain images through its image property.

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✅ Changes

Affected file : await_images.dart

• Replaced the if (decoration is BoxDecoration) check with a switch expression to handle multiple decoration types.
• Added support for extracting images from ShapeDecoration.
• Defaulted to null when the decoration type is not handled or contains no image.

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🧪 Motivation

Some widgets use ShapeDecoration instead of BoxDecoration to apply background images. Without this change, those images were not detected and precached during widget tests, potentially leading to rendering issues or test flakiness.

By extending support to ShapeDecoration, this PR ensures that all relevant images are properly precached, improving the reliability and coverage of widget tests.

[T-moz]

@remax21 Thanks for the contribution !

Sorry for the delay, I din't saw your pull request

[T-moz]

@remax21 It's now available in the 0.9.1 version