Fix model download status race conditions and integrity checks

[a-ghorbani]

Description

Two probable race conditions were identified in the model download process:

1. Partially downloaded files could be marked as "downloaded" due to file existence checks
2. Hash calculations could occur on incomplete files, leading to incorrect integrity checks

Solution
Modified the download process to:

1. Track active downloads and prevent marking files as downloaded during download
2. Clear hash at download start to force recalculation
3. Only set isDownloaded and hash after confirmed successful download

Fixes #163
Fixes #164

Platform Affected

• iOS
• Android

Checklist

• Necessary comments have been made.
• I have tested this change on:
  • iOS Simulator/Device
  • Android Emulator/Device
• Unit tests and integration tests pass locally.