Fix iOS/Android test infrastructure

[wmathurin]

Summary

• Fix prepareios.js: stop overwriting tracked RCTTest/ files (customized for bridgeless mode)
• Fix CI: add execute permission to create_test_credentials_from_env.js
• Simplify test credentials: both iosTests/ and androidTests/ expect test_credentials.json at their root (removed shared/test/ directory and symlink)
• Use file:.. for react-native-force in test package.json — tests always run against local code
• Add Gradle task to copy test_credentials.json to assets at build time (no more ordering dependency with prepareandroid.js)
• Fix react-android/hermes-android version: 0.81.5 → 0.82.1
• Update .gitignore for test artifacts

Test plan

• iOS tests build and pass (35/35) via Xcode
• prepareandroid.js runs successfully
• Android project builds in Android Studio
• CI runs create_test_credentials_from_env.js before prepare scripts
• Missing test_credentials.json prints a clear warning instead of silently failing

[github-actions]

	Tests	Passed ✅	Skipped	Failed
iOS ^26 Test Results	35 ran	35 ✅

Test	Result
No test annotations available

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.95%. Comparing base (eec0c3a) to head (19aa485).
⚠️ Report is 22 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #453      +/-   ##
==========================================
+ Coverage   72.62%   76.95%   +4.32%     
==========================================
  Files          13       15       +2     
  Lines         559      651      +92     
==========================================
+ Hits          406      501      +95     
+ Misses        153      150       -3     

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[github-actions]

	Tests	Passed ✅	Skipped	Failed
iOS ^18 Test Results	35 ran	35 ✅

Test	Result
No test annotations available