feat: support concurrent chunk uploads

[TorstenDittmann]

This PR updates the SDK to support concurrent chunk uploads.

[greptile-apps]

Greptile Summary

This PR rewrites the chunked upload path in Client.swift to support concurrent chunk uploads using Swift's withThrowingTaskGroup, replacing the previous sequential while loop. A sliding-window scheduler dispatches up to Client.maxConcurrentUploads (now a configurable static property, defaulting to 8) chunks at a time.

• The first chunk is always uploaded serially to obtain the server-assigned file ID before concurrent tasks are dispatched — correct for Appwrite's resumable upload protocol.
• Resumable uploads are handled: an existing file check sets offset and result = map before the task group runs, fixing a latent crash where a fully-pre-uploaded file would have called converter!([String:Any]()).
• completedResponse ?? lastChunkResponse ?? result provides a fallback chain so the function always returns the most complete response available.

Confidence Score: 4/5

The concurrent upload rewrite is structurally sound with no data-loss paths; the main gap is that onProgress is never fired when resuming an already-complete file.

The sliding-window task group and first-chunk serialisation are implemented correctly. The addition of result = map fixes a latent crash for pre-completed files. The one unresolved behavioural gap is that onProgress is never fired when the upload resumes into an already-complete state, which could leave callers silently waiting.

Sources/Appwrite/Client.swift — the post-task-group onProgress gap and uploadedBytes initialisation clarity in the first-chunk block.

Important Files Changed

Filename	Overview
Sources/Appwrite/Client.swift	Chunked upload rewritten to use concurrent task group with sliding-window dispatch; first chunk serialised for ID acquisition; resumable-upload path correctly seeds result from existence check; maxConcurrentUploads exposed as configurable static.

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