[ben-milanko]

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

Feature: per-caller request cancellation. This is the second of the two pull requests suggested in the review of #504. Web persistence is in a separate PR.

⤵️ What is the current behavior?

A download cannot be cancelled once requested. A widget that is disposed, or scrolled away, still waits for its download to finish.

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

• getFileStream, getSingleFile, downloadFile and ImageCacheManager.getImageFile take an optional CancellationToken. Cancelling ends that call with a CancelledException.
• Cancellation applies per caller. Callers of the same url share one download, and each gets its own stream. The download, and a shared resize, stops only once every caller has cancelled. A caller without a token keeps the download running, and one caller's token never fails or is ignored for another caller.
• A cancelled or failed download deletes its partial file. Downloads go to a fresh path, so the previously cached file stays valid.
• The download sink is closed when the response stream errors, as the old .pipe(sink) did.

💥 Does this PR introduce a breaking change?

No. FileService.get and BaseCacheManager keep their exact signatures. The token reaches the file service as CancellationToken.current, a zone value. HttpFileService aborts its request on it, and a custom FileService that ignores it is simply abandoned and its late response body released. flutter_cache_manager_firebase analyzes and passes its tests unchanged against this branch, without a dependency override.

🐛 Recommendations for testing

• flutter test: 168 tests pass, including a new test/cancellation_test.dart. It covers mid-stream cancellation, a response stream error, two callers on one url (in both orders), every caller cancelling, a queued request, an already-cancelled token, a service that ignores the token, and the token passing through getFileStream, getSingleFile, downloadFile and getImageFile, with and without resizing.
• test/mock.mocks.dart is regenerated for the new optional parameter on WebHelper.downloadFile.
• flutter build web --wasm in example/ builds.

📝 Links to relevant issues/docs

Follows the review of #504.

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current main
• Version bumped and dated CHANGELOG entry added

Both this PR and the web persistence PR bump to 3.6.0. Whichever merges second will move to the next minor version when rebased.