refactor: resource fetcher

[chmjkb]

Description

• Extracted shared base class (BaseResourceFetcherClass) into the core package containing the fetch loop, source dispatching, and pause/resume/cancel plumbing — eliminating duplication between expo and bare
  implementations
• Extracted platform-specific handlers (handlers.ts) for both packages, separating download mechanics from the class structure
• Rewrote both fetchers as classes extending the base, implementing only platform-specific file system and download operations
• Replaced the linked-list download pattern with a simple sequential for loop
• Cancel now throws DownloadInterrupted instead of returning null, giving callers a consistent error-based API

Note: this PR still uses the legacy FS as the support for background / resumable downloads is WIP.

Introduces a breaking change?

• Yes
• No

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

1. Create a demo app trying to run all the functionalities exposed in Resource Fetchers
2. Fetch function with single and/or multiple resources
3. Cancel long fetching file, also verify fetch throws an error when its canceled
4. Check if a model with downloaded file works
5. Make sure deleting resources work
6. Try running a background download

Screenshots

Related issues

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings

Additional notes

[msluszniak]

Please run yarn lint --fix in the root of the package and push changes

[msluszniak]

I performed detailed testing, code available on branch: https://github.com/msluszniak/react-native-executorch/tree/test/pr-1023-resource-fetcher

Everything worked correctly.