Fix React-CoreModules missing React-featureflags header path under use_frameworks!

[radko93]

Summary:

React/CoreModules/RCTRedBox.mm imports <react/featureflags/ReactNativeFeatureFlags.h> (added in #56574). Under use_frameworks! this fails to build:

RCTRedBox.mm:19:9: 'react/featureflags/ReactNativeFeatureFlags.h' file not found
Did not find header 'featureflags/ReactNativeFeatureFlags.h' in framework 'react'

React-CoreModules.podspec uses plain s.dependency "React-featureflags", which does not add the framework's Headers dir to HEADER_SEARCH_PATHS. Switching to add_dependency(s, "React-featureflags") preserves the same pod dependency and also adds the dependency header path, matching every neighboring dep (React-debug, React-runtimeexecutor, React-jsinspector*, etc.) in the same file.

On main, this missing podspec-level header path is currently likely masked by the global update_search_paths entry added in #55679, which injects React_featureflags.framework/Headers into inherited header search paths for use_frameworks! builds. 0.83-stable does not have that global entry, so the same podspec issue surfaces as a build regression in 0.83.7.

This change fixes the dependency at the target that directly imports the header, so React-CoreModules no longer relies on a broad inherited header path. A cherry-pick to 0.83-stable would fix the regression in 0.83.7.

Changelog:

[IOS] [FIXED] - Fix React-CoreModules failing to compile with use_frameworks! due to missing React-featureflags header path

Test Plan:

RN 0.83.7 consumer app with use_frameworks! :linkage => :static, RCT_USE_RN_DEP=1: fails before this change. After the fix, pod install regenerates React-CoreModules.debug.xcconfig so HEADER_SEARCH_PATHS includes:

${PODS_CONFIGURATION_BUILD_DIR}/React-featureflags/React_featureflags.framework/Headers

Verified locally that the React-CoreModules pod target builds after the fix:

xcodebuild -project ios/Pods/Pods.xcodeproj \
  -scheme React-CoreModules \
  -configuration Debug \
  -sdk iphonesimulator \
  -derivedDataPath /tmp/rncoremodules-test \
  CODE_SIGNING_ALLOWED=NO build

[meta-codesync]

@cipolleschi has imported this pull request. If you are a Meta employee, you can view this in D102328650.

[meta-codesync]

@cipolleschi merged this pull request in fb0d28c.

[react-native-bot]

This pull request was successfully merged by @radko93 in fb0d28c

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