Adds debugLabel for the image stream completer

[AlexV525]

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

This is a bug fix. Fixes #973

This also helps to display the info in the Flutter Inspector.

As a reference, this is how the MultiFrameImageStreamCompleter implements the debugLevel.
https://github.com/flutter/flutter/blob/574b7e765d86f3f70eac8817a2906ac3099c0bf9/packages/flutter/lib/src/painting/image_stream.dart#L967

⤵️ What is the current behavior?

======== Exception caught by painting library ======================================================
The following message was thrown while painting an image:
Image null has a display size of 794×529 but a decode size of 1956×1280, which uses an additional 10849KB (assuming a device pixel ratio of 2.625).

Consider resizing the asset ahead of time, supplying a cacheWidth parameter of 794, a cacheHeight parameter of 529, or using a ResizeImage.

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

======== Exception caught by painting library ======================================================
The following message was thrown while painting an image:
Image https://example.com/redact.jpeg has a display size of 794×529 but a decode size of 1956×1280, which uses an additional 10849KB (assuming a device pixel ratio of 2.625).

Consider resizing the asset ahead of time, supplying a cacheWidth parameter of 794, a cacheHeight parameter of 529, or using a ResizeImage.

💥 Does this PR introduce a breaking change?

No.

[rickdijk]

This repository now uses main as its only branch, so I have retargeted this PR from develop to main. Nothing about the review changes. If GitHub now shows a conflict, a rebase onto main will clear it, and we will follow up on that separately.

[AlexV525]

@rickdijk PTAL

[rickdijk]

Sure, here's a few comments:

• adhere to contributing guidelines by adding a CHANGELOG entry as well as a version bump in pubspec.yaml
• Not a blocker, but nice to have and easy to write: please add a test. Something like: build a MultiImageStreamCompleter with a label and check that ImageInfo.debugLabel matches

[AlexV525]

@rickdijk Addressed in a04c3d6, with the branch rebased onto main. PTAL.

• CHANGELOG.md and pubspec.yaml: entry added and version bumped to 4.0.3 — 4.0.2 is already tagged on main, so this fix rides the next patch.
• Tests added:
  • test/image_stream_completer_test.dart: a MultiImageStreamCompleter built with a debugLabel emits ImageInfos whose debugLabel matches, covering both the static-image and the animated-image emit paths.
  • test/image_provider_test.dart: resolving a CachedNetworkImageProvider emits an ImageInfo whose debugLabel is the URL, which is the behavior from the issue report.

All three tests fail without the fix and pass with it. From cached_network_image/: dart format --set-exit-if-changed ., flutter analyze and flutter test (40 tests) are green.

[rickdijk]

Apply the change and this PR is approved when it completes CI

[AlexV525]

@rickdijk Thanks. CI is green so I've applied the suggestion.