fix: JSC support — createJSRuntimeFactory compile error & Hermes pods not excluded

[tux2nicolae]

Changelog:

[iOS] [Fixed] - Fix compilation error in createJSRuntimeFactory and skip Hermes pods when USE_THIRD_PARTY_JSC is enabled

Summary

Two fixes for third-party JSC engine support (USE_THIRD_PARTY_JSC=1):

1. createJSRuntimeFactory missing return path

When USE_THIRD_PARTY_JSC is set to 1, the #if USE_THIRD_PARTY_JSC != 1 block in createJSRuntimeFactory is skipped entirely. Since there is no #else branch, the method — which returns JSRuntimeFactoryRef (a non-void type) — has no return statement on that code path, causing a compilation error.

Fix: Added an #else branch that raises an NSException (indicating subclasses must override this method) and returns nil to satisfy the compiler.

2. Hermes pods always installed despite USE_THIRD_PARTY_JSC=1

The hermes_enabled flag in use_react_native! (react_native_pods.rb) is hardcoded to true, completely ignoring the USE_THIRD_PARTY_JSC environment variable. This causes hermes-engine, React-hermes, and React-RuntimeHermes to always be installed — even when a third-party JSC engine is configured — bloating the binary with an unused JS engine.

Fix: Changed hermes_enabled= true to hermes_enabled= !use_third_party_jsc() so Hermes pods are excluded when USE_THIRD_PARTY_JSC=1.

Fixes #54268

Changes

• RCTDefaultReactNativeFactoryDelegate.mm: Added #else branch to createJSRuntimeFactory
• react_native_pods.rb: Set hermes_enabled based on use_third_party_jsc() instead of hardcoding true

Motivation

Projects using third-party JSC (e.g. via @react-native-community/javascriptcore) set USE_THIRD_PARTY_JSC=1. Without these fixes:

1. The code fails to compile because the non-void function createJSRuntimeFactory has no return statement in the JSC path
2. Hermes engine pods are still installed and bundled, adding unnecessary binary size — particularly problematic for iOS App Clips which have a strict 15MB size limit

Test Plan

• Verified pod install with USE_THIRD_PARTY_JSC=1 correctly excludes hermes-engine, React-hermes, and React-RuntimeHermes (130 pods instead of 133)
• Verified Podfile.lock no longer references Hermes pods
• Verified the project compiles successfully with USE_THIRD_PARTY_JSC=1
• Verified the project compiles successfully without USE_THIRD_PARTY_JSC (default Hermes path unchanged)

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