fix(mobile): fix flutter cache eviction on thumbnails

[LeLunZ]

Description

In #27624 we fixed that requests get evicted from cache too late. When creating the PR I was too invested in checking that we don't do double evictions anywhere anymore and forgot that the FullImageProvider isn't actually everything and especially not what we use for thumbnails. The flag is missing from thumbhash providers and "normal" ImageProviders (which we use for thumbnails).
That currently results in the cache always being evicted for these images, after the corresponding widget is disposed, even if they are fully loaded.

Setting isFinished manually everywhere we need, is kind of a hassle and easily forgotten. (as noticed through this PR)
Thats why I made it part of the loadRequest methods, so it's part of the whole CancellableImageProviderMixin contract. And you actually have to opt out of setting the flag, which the "special" providers then have to do (eg. FullImageProvider).
Which means ThumbhashProvider and "normal" image provider don't have to set the flag.

@mertalev Sorry for the inconvenience.

Fixes #27679

How Has This Been Tested?

• verified that the flag is now actually set to true (debugged this)

Checklist:

Please describe to which degree, if any, an LLM was used in creating this pull request.

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[LeLunZ]

bad rebase

[LeLunZ]

actually @mertalev in the iOS code we have this:

 if let error = error {
      if request.isCancelled || (error as NSError).code == NSURLErrorCancelled {
        return request.completion(ImageProcessing.cancelledResult)
      }
      return request.completion(.failure(error))
    }

So we also return null incase the request errors with NSURLErrorCancelled, but I don't really know if that only happens if request.cancel is invoked, or if the system can also can requests

[LeLunZ]

Actually returning a cancellation incase the error code is NSURLErrorCancelled while isCancelled is false, doesn't seem right.
As this should still be an error, as we ourselves didn't invoke the cancellation

[mertalev]

There is no case where the request is cancelled but we didn't request it. It's a slightly redundant check.

[LeLunZ]

Yeah I guess then this is finished?