feat: add comprehensive async tests and remove orientation variable i…

[CaiJingLong]

This pull request adds comprehensive async tests for the ImageSizeGetter library and refines the JPEG decoder logic to improve robustness and correctness. The main focus is on ensuring that asynchronous image size detection works reliably across a variety of formats and edge cases, while also improving the decoder's handling of JPEG files.

Testing improvements

• Added a new group of async tests in image_size_getter_test.dart to cover getSizeAsync and getSizeResultAsync for multiple image formats (GIF, JPEG, PNG, WebP, BMP), including extended and lossless WebP formats. These tests verify correct size detection and decoder identification.
• Added tests for error handling, including cases for non-existent files and unsupported formats, ensuring that the async methods throw appropriate exceptions.
• Added tests for async size detection using memory input, confirming that in-memory images are processed correctly.

JPEG decoder improvements

• Improved the JPEG decoder by explicitly checking for the App1 block, which is often used for EXIF metadata and orientation, increasing robustness for non-standard JPEGs.
• Removed incorrect orientation extraction from the JPEG decoder, ensuring that orientation is handled properly and not set from an invalid byte position.

[Copilot]

Pull Request Overview

This PR adds comprehensive async testing for the image_size_getter package and removes an unused orientation variable assignment in the JPEG decoder implementation.

• Adds extensive async test coverage for all supported image formats (GIF, JPEG, PNG, WebP, BMP)
• Includes error handling tests for non-existent files and unsupported formats
• Removes unused orientation variable assignment in JPEG decoder

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
packages/image_size_getter/test/image_size_getter_test.dart	Adds comprehensive async test suite covering all image formats and error scenarios
packages/image_size_getter/lib/src/decoder/impl/jpeg_decoder.dart	Removes unused orientation variable assignment and adds explanatory comment