Move RN iOS tests into XcodeGen integration app

[kieran-osgood-shopify]

Stack Context

This is the first PR in a two-PR stack that separates React Native iOS test relocation from the Expo sample migration.

Stack:

1. Move React Native iOS bridge tests into an XcodeGen/CocoaPods integration app.
2. Migrate the React Native sample to Expo prebuild/dev-client.

What?

• Adds platforms/react-native/test/rct-integration-app as a small XcodeGen-generated CocoaPods host app for RN iOS tests.
• Moves the RN iOS bridge/view tests out of the generated sample iOS project and into the new integration app.
• Moves the protocol relay tests into the same RN iOS test target and removes the separate nested SwiftPM test package.
• Extends platforms/swift/Scripts/generate_xcode_projects so dev up generates the RN RCT integration app through the existing XcodeGen pipeline.
• Updates dev rn test ios to run the Xcode/CocoaPods integration tests.

Why?

The Expo migration deletes the committed generated React Native sample native projects. Before that happens, the RN-specific iOS behavioral coverage needs a stable home that is not inside generated sample output.

This keeps the tests running against the real React Native/CocoaPods integration until React Native SPM support is ready.

Validation

• dev rn test ios
• dev rn test js
• dev rn lint
• git diff --check

[kieran-osgood-shopify]

• Migrate React Native sample to Expo prebuild #297
• Move RN iOS tests into XcodeGen integration app #322 👈 (View in Graphite)
• main

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