refactor(mobile): iOS introduce ImageRequest base class with unified cancellation and finish helpers

[LeLunZ]

Description

After PR #27486, LocalImageRequest and RemoteImageRequest each still duplicated the same boilerplate: an isCancelled flag, a stored callback, a cancel() method, and identical malloc/vImage_Buffer blocks in their respective ImageApiImpl.

This PR adds the ImageRequest base class with:

• Thread-safe cancellation, and guarantee that it is only cancelled once
• cancel(): sets isCancelled, nil's the callback, calls onCancel(), then fires cancelledResult
• onCancel(): empty hook for subclasses to cancel their underlying work (operation, URL task).
• finish(with:): completes the request with any result.
• finish(encoding:): allocates a raw pointer from Data, frees it if cancelled, otherwise completes with pointer + length
• finish(cgImage:format:): takes format as inout so the caller's static var is passed by reference — only one copy occurs when forwarding to vImage_Buffer. Frees the buffer if cancelled

The Remote and Local ImageRequest implement the Base class.

Behaviour changes that this PR includes:
the pigeon callback is now called directly when setting isCancelled to true. Not when the thread runs and sees that isCancelled is true and terminates the operation.
I think we can already communicate to flutter that the operation is cancelled and we don't have to wait on the thread to actually return or?

How Has This Been Tested?

• on an iOS simulator

Checklist:

• I have carefully read CONTRIBUTING.md
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if applicable
• I have no unrelated changes in the PR.
• I have confirmed that any new dependencies are strictly necessary.
• I have written tests for new code (if applicable)
• I have followed naming conventions/patterns in the surrounding code
• All code in src/services/ uses repositories implementations for database calls, filesystem operations, etc.
• All code in src/repositories/ is pretty basic/simple and does not have any immich specific logic (that belongs in src/services/)

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

/

[immich-push-o-matic]

Label error. Requires exactly 1 of: changelog:.*. Found: 📱mobile. A maintainer will add the required label.

[LeLunZ]

@mertalev Regarding this being nullable, thats how the pigeon communication is typed. null values means cancellation.
Thats also the same on main. See here