fix: prevent image decode future from completing twice - #520

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Bug fix.

⤵️ What is the current behavior?

When ImageCacheManager resizes a multi-frame image, such as an animated PNG,
_decodeImage keeps its ImageStreamListener registered after receiving the
first frame.

Subsequent frames call Completer.complete(...) again, causing:

StateError: Bad state: Future already completed

🆕 What is the new behavior (if this is a feature change)?

_decodeImage now removes its listener after the first frame and guards
success/error completion with Completer.isCompleted.
Animated PNGs can be resized without throwing Future already completed.

💥 Does this PR introduce a breaking change?

No.

🐛 Recommendations for testing

1. Use a CacheManager with ImageCacheManager.
2. Request a resized animated PNG by passing maxWidth and/or maxHeight.
3. Confirm that subsequent animation frames do not throw an exception.
   Example APNG:
   https://upload.wikimedia.org/wikipedia/commons/1/14/Animated_PNG_example_bouncing_beach_ball.png
   A local APNG fixture is recommended for an automated regression test.

CachedNetworkImage(
      imageUrl: 'https://upload.wikimedia.org/wikipedia/commons/1/14/Animated_PNG_example_bouncing_beach_ball.png',
      width: 100,
      height: 100,
      maxWidthDiskCache: 256,
      maxHeightDiskCache: 256,
)

📝 Links to relevant issues/docs

Fixes #400

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current develop