test(images): assert that a cached image is served, not that something happened

[m4bard]

Fixes #897.

Nine of the twelve ImagesController test files ended with the same shape:

if (result is PhysicalFileResult fileResult)
{
    Assert.Equal(fullPath, fileResult.FileName);
}
else
{
    Assert.IsType<NotFoundObjectResult>(result);
}

Both branches pass, so the test succeeded whether the image was served or the endpoint returned 404. Two of the nine say in their own names that they cover serving, GetImage_TriggersMetadataDownload_AndServesCachedImage and GetImage_WhenTempExists_AndAudiobookExists_MovesToLibraryAndServesLibraryFile, and both passed with serving removed. What was covered was the lookup path.

Each of the nine creates the file it then asks for, so serving it is the only correct outcome and the permissive form was not buying anything. They now assert the type outright.

The comment they carried, "Expect either PhysicalFileResult when file exists or NotFound if it wasn't found", described a condition the test itself controls. It is replaced with a note on why the strict form is the right one.

The check

I re-ran the control from the issue: the cached-image return in ImageResponseBuilder replaced with a flat 404, so every path through the builder fails.

Before this change, 2 of 12 failed. After it, 11 of 12 fail. The twelfth is ImagesController_PlaceholderFallbackTests, which asserts on the placeholder return further down and is correctly out of scope for that control.

Each test file already uses its own content root under the temp directory, so asserting strictly does not make them depend on each other's order.

No production code changes.

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Worked through with Claude Code at my direction. The claims above were checked by running them rather than by reading, and I reviewed this before posting.