Move synchronous image upload preparation off the main actor (#6323)

* Move synchronous image upload preparation off the main actor

Signed-off-by: Ivan Magda <imagda15@gmail.com>

* Make media preparation cooperative with cancellation

Replace the dispatch queue with concurrent copy, image and video-thumbnail
functions. Return sendable upload metadata directly, bound each batch to
two preparations and discard unfinished results when cancellation occurs.

Keep file metadata reads synchronous and prevent the preview from sending
a result after its preparation task has been cancelled. Replace unrelated
tests with cancellation and batch-order coverage requested during review.

* Update MediaUploadingPreprocessor.swift

* Run media preparation in a nonisolated preprocessor

Make MediaUploadingPreprocessor a nonisolated struct whose copy, image
processing and video thumbnail run in @concurrent instance methods,
instead of a main-actor type with static helpers and an extra setting
parameter.

Check cancellation with plain early returns between the slow steps,
name the batch limit, stop logging cancelled video exports as errors,
and restore develop's structure and doc comments where they were changed
without need.

Replace the tests that didn't reach the new code with tests for cleanup
on failure, on batch failure and on cancellation, and make the preview
test set up its blocking mock before the view model starts processing.

* Apply review suggestions to media preparation

Docs:
- Drop the doc comment on maximumConcurrentProcessingCount, the name says it.
- Move the cleanup note into the main description of both processMedia
  functions, so the Returns lines match develop again.

Tests:
- Rename the test helper to copyResourceToTemporaryDirectory(_:).

---------

Signed-off-by: Ivan Magda <imagda15@gmail.com>
Co-authored-by: Ivan Magda <imagda15@gmail.com>

UnitTests/Sources/MediaUploadPreviewScreenViewModelTests.swift

@@ -34,6 +34,35 @@ final class MediaUploadPreviewScreenViewModelTests {
         userIndicatorController = UserIndicatorControllerMock()
     }
     
+    @Test
+    func cancelWhileSendWaitsForPreparation() async throws {
+        // Given a screen whose media preparation is still running.
+        let (startedStream, startedContinuation) = AsyncStream<Void>.makeStream()
+        let (resumeStream, resumeContinuation) = AsyncStream<Void>.makeStream()
+        defer { resumeContinuation.finish() }
+        
+        let preparationStarted = deferFulfillment(startedStream) { _ in true }
+        setUpViewModel(urls: [imageURL], expectedCaption: nil) {
+            startedContinuation.yield()
+            var iterator = resumeStream.makeAsyncIterator()
+            _ = await iterator.next() // Only returns once the preparation is cancelled.
+            return .success(100 * 1024 * 1024)
+        }
+        try await preparationStarted.fulfill()
+        
+        // When sending and then dismissing the screen before the preparation finishes.
+        context.send(viewAction: .send)
+        let loadingFinished = deferFulfillment(context.observe(\.viewState.shouldDisableInteraction)) { !$0 }
+        context.send(viewAction: .cancel)
+        viewModel.stopProcessing() // Called by the coordinator when the screen is dismissed.
+        try await loadingFinished.fulfill()
+        
+        // Then nothing is sent and no error is shown.
+        #expect(timelineProxy.sendImageUrlThumbnailURLImageInfoCaptionRequestHandleCallsCount == 0)
+        #expect(userIndicatorController.submitIndicatorDelayCallsCount == 1, "Only the loading indicator should be shown")
+        #expect(context.viewState.bindings.alertInfo == nil)
+    }
+    
     @Test
     func imageUploadWithoutCaption() async throws {
         setUpViewModel(urls: [imageURL], expectedCaption: nil)
@@ -293,7 +322,8 @@ final class MediaUploadPreviewScreenViewModelTests {
                                 expectedCaption: String?,
                                 maxUploadSizeResult: Result<UInt, ClientProxyError>? = nil,
                                 simulateGallerySendFailure: Bool = false,
-                                galleryEnabled: Bool = true) {
+                                galleryEnabled: Bool = true,
+                                maxUploadSizeClosure: (() async -> Result<UInt, ClientProxyError>)? = nil) {
         timelineProxy = TimelineProxyMock(.init())
         timelineProxy.sendAudioUrlAudioInfoCaptionRequestHandleClosure = { [weak self] _, _, caption, _ in
             self?.verifyCaption(caption, expectedCaption: expectedCaption) ?? .failure(.sdkError(TestError.unknown))
@@ -316,6 +346,7 @@ final class MediaUploadPreviewScreenViewModelTests {
         if let maxUploadSizeResult {
             clientProxy.underlyingMaxMediaUploadSize = maxUploadSizeResult
         }
+        clientProxy.maxMediaUploadSizeClosure = maxUploadSizeClosure
         
         viewModel = MediaUploadPreviewScreenViewModel(mediaURLs: urls,
                                                       caption: nil,