[paulb777]

Problem

data(maxSize:), putDataAsync, putFileAsync, and writeAsync wrap the callback APIs in
withCheckedThrowingContinuation and ignore Swift task cancellation. Cancelling the calling task
(for example, when the view of a SwiftUI .task modifier disappears) doesn't stop the transfer:
an upload still completes, and the caller stays suspended until the transfer finishes. These
APIs don't return the StorageTask, so callers can't cancel the transfer themselves.

An earlier attempt, #8529, stalled because onCancel can run before the Storage task exists,
and the fix proposed there needed new public API.

Change

• The four APIs start their transfer inside withTaskCancellationHandler. onCancel calls
  cancel() on the StorageUploadTask or StorageDownloadTask, which stops the request and
  fails the task with StorageError.cancelled, and that error is thrown to the caller.
• A small lock-protected StorageTaskCanceller covers every ordering. If onCancel runs before
  the task is created (it runs first when the calling task is already cancelled), the task is
  cancelled as soon as it's created.
• Results always come through the completion handler APIs, which register their success and
  failure observers before the task starts. The onProgress variants used to add .success and
  .failure observers after starting the task. Now they only add a .progress observer, which
  also takes them off the late-observer path in Storage task observers can receive duplicate terminal callbacks #16813 (fixed for all callers by fix(storage): deliver terminal task events to late observers only once #16820).
• cancel() does nothing once a task has succeeded or failed, so a late cancellation can't
  resume the continuation a second time.
• No public API changes: the new helpers are private. getMetadata, updateMetadata,
  delete, list, and downloadURL are unchanged, since their internal tasks have no cancel().

Testing

Added StorageAsyncAwaitTests:

• Six cancellation tests: data(maxSize:) (cancelled during the request, and with an
  already-cancelled task), putDataAsync with and without onProgress, putFileAsync, and
  writeAsync. Each uses a test block that never responds and cancels the calling task once the
  request has started. It checks that StorageError.cancelled is thrown within 10 seconds and
  that the request was stopped (isFetching == false).
• Four regression tests for the success paths, including that onProgress is still called.

xcodebuild -scheme FirebaseStorageUnit test -destination 'platform=macOS' \
  -only-testing:FirebaseStorageUnit/StorageAsyncAwaitTests

• Without the source change (tests only): the six cancellation tests fail with Asynchronous wait failed: Exceeded timeout of 10 seconds, with unfulfilled expectations: "finished", and the five
  that cancel an in-flight request also fail with The request was not stopped. The four success
  tests pass.
• With the change: all 10 pass.
• Full suite xcodebuild -scheme FirebaseStorageUnit test -destination 'platform=macOS':
  173 tests, 0 failures.

Note: existing leak after cancel()

While testing this, I confirmed an existing leak that this change makes easier to hit. After
StorageTask.cancel(), upload and download tasks are never deallocated, including the data passed
to putData. Today, only an explicit cancel() hits the leak. With this PR, cancelling the calling
task hits it too.

#16841 fixes the leak separately, so ideally it lands first. The two PRs only overlap in the
CHANGELOG.

Fixes #11786

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[paulb777]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces Swift task cancellation support to FirebaseStorage's asynchronous APIs, including data(maxSize:), putDataAsync, putFileAsync, and writeAsync. When a calling Swift task is cancelled, the underlying storage operation is cancelled and a StorageError.cancelled error is thrown. This behavior is managed by a new thread-safe StorageTaskCanceller helper class and the withCancellableStorageTask function. Additionally, unit tests have been added to verify cancellation and success paths. No review comments were provided, so there is no feedback to address.