Fix data race on SessionDataTask.started and tighten concurrency tests

[onevcat]

Summary

Follow-up to #2539 (thanks @devzahirul), which fixed the unlocked callbacksStore iteration in forceCancel() and is now merged.

• Fix the same class of race on started, found while reviewing Fix data race in SessionDataTask.forceCancel() #2539: resume() wrote the flag unsynchronized while ImageDownloader.startDownloadTask reads it from other threads. It is now guarded by the existing lock, with resume() performing an atomic check-and-set and calling task.resume() outside the lock.
• Condense the forceCancel comment from Fix data race in SessionDataTask.forceCancel() #2539 to the two load-bearing facts (cross-thread mutation; cancel(token:) re-acquires the non-recurrent lock).
• Tighten the concurrency tests: instead of adding more single-purpose stress tests, add one combined testSessionDataTaskConcurrentAccessIsThreadSafe that hammers all mutating and reading surfaces of SessionDataTask concurrently (addCallback, forceCancel, resume, started, containsCallbacks, didReceiveData, mutableDataCount, completeAndRemoveAllCallbacks). The focused Fix data race in SessionDataTask.forceCancel() #2539 regression test remains as-is.

Verification

Before the fix, the started race is reported deterministically by Thread Sanitizer (process crashes):

WARNING: ThreadSanitizer: data race
  Write of size 1 by thread T3:
    #0 Kingfisher.SessionDataTask.started.setter

After the fix, both the combined test and the #2539 regression test run clean under TSan:

xcodebuild test -project Kingfisher.xcodeproj -scheme Kingfisher \
  -destination 'platform=macOS' -enableThreadSanitizer YES \
  -only-testing:KingfisherTests/ImageDownloaderTests/testSessionDataTaskConcurrentAccessIsThreadSafe \
  -only-testing:KingfisherTests/ImageDownloaderTests/testForceCancelConcurrentWithAddCallbackIsThreadSafe

Full macOS suite passes (666 tests).

Ref: #2539