test(images): measure the thumbnail decode without imageBuilder

[vannt-dev]

Unblocks the v1.6.0 release: the push of chore(release): 1.6.0 to main failed
its Android integration job (run 33761887174),
so release-candidate and publish-release were skipped and no tag was cut.

What actually broke

The failure was in the test, not the app.

CachedNetworkImage implements memCacheWidth by handing it to OctoImage,
which wraps the provider in ResizeImage.resizeIfNeeded
(octo_image/lib/src/image/image.dart:205). But when an imageBuilder is
given, _octoImageBuilder throws away the widget OctoImage built and calls
imageBuilder(context, _image) with the raw CachedNetworkImageProvider
(cached_network_image-3.4.1/lib/src/cached_image_widget.dart:299).

The test used imageBuilder purely to detect that loading had finished, then
painted Image(image: provider) — decoding a second, full-resolution copy. Its
assertion read imageCache.currentSizeBytes, a global total, which then held
both bitmaps:

full      = 5_832_000  = 1080 x 1350 x 4
thumbnail = 6_065_280  = 5_832_000 + (216 x 270 x 4)

Whether it passed came down to which decode reached the cache first — it passed
on the PR run for the very same commit and failed on the push to main.

The fix

The app never passes imageBuilder — all six memCacheWidth call sites use a
plain CachedNetworkImage — so the measurement now loads the image the way
production does and completion is detected from the cache itself. One decode,
one number:

full      = 5_832_000
thumbnail =   233_280   (216 x 270 x 4)

Verification

Run on a local Aegis_API_34 emulator against tool/fixture_server.js, the
same pair CI runs — twice, including once on the exact post-dart format file:

• integration_test/android_storage_test.dart + integration_test/slideshow_render_test.dart — 20/20 passed, both runs, identical byte counts
• flutter test — 261 passed, 8 skipped
• flutter analyze — no issues
• dart format --set-exit-if-changed lib test integration_test — clean

No production code is touched.

Once this lands on main, release-candidate takes its "Version is unchanged;
checking for an interrupted release" path (.github/workflows/ci.yml:186):
pubspec.yaml is still 1.6.0, CHANGELOG.md has valid [1.6.0] notes, and
v1.6.0 does not exist yet — so the release publishes without another bump.