…erForData: (#57297)

Summary:
`imageURLLoaderForURL:` had a broken double-checked lock on `_loaders`: the outer `if (!_loaders)` guard and the `for` loop iteration both ran without `_loadersMutex`, while the assignment ran inside it. `imageDataDecoderForData:` had no lock at all on `_decoders`. Both methods write the ivar in two steps (raw list then sorted), so a concurrent reader could observe the intermediate value and iterate a concurrently-released array, crashing with `EXC_BAD_ACCESS` in `objc_release` on a GCD worker thread.

Verified with a reproducer — crashes 5/5 without the fix, 0/10 with it: https://github.com/mfazekas/rn-rctimageloader-dcl-repro

## Fix

Build the loader and decoder lists once in `setUp` (alongside the URL request queue), under `_setupLock` and gated by the existing `_didSetup` atomic flag. Every entry point calls `setUp` before reading the now write-once `_loaders` and `_decoders`, so a reader can never observe a half-built or concurrently-released array.

This replaces the broken double-checked lock where the outer `if (!_loaders)` guard and the `for` loop iteration ran outside the mutex.

## Changelog:

[iOS] [Fixed] - Fix a data race in `RCTImageLoader` loader and decoder lazy initialization that could crash with `EXC_BAD_ACCESS`

## Related

Fixes #57296
See also #46115, #46153

Pull Request resolved: #57297

Reviewed By: javache

Differential Revision: D109404243

Pulled By: fabriziocucci

fbshipit-source-id: 359d4c70e2cbe1bc70a7e6a1dadc3e3b89c59ff4