⚡ Make marketplace bundle download non-blocking

[miethe]

💡 What:
Wrapped the blocking synchronous file download operation (broker_inst.download) in asyncio.to_thread.

🎯 Why:
The marketplace bundle download uses requests and file I/O which are synchronous and blocking. When executed directly within an async endpoint, it blocks the entire asyncio event loop, causing the server to become unresponsive to other requests until the download completes. Offloading this to a thread ensures the event loop remains responsive.

📊 Measured Improvement:
Verified using a reproduction script that simulated a slow download (1.2s).

• Before: The event loop was blocked for the entire duration (1.2s), preventing other tasks from running.
• After: The event loop continued to tick approximately every 0.1s during the download, confirming non-blocking behavior.

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PR created automatically by Jules for task 12664104238556278033 started by @miethe

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