fix: JSC support — createJSRuntimeFactory compile error & Hermes pods not excluded (facebook#55817)

Summary:
## Changelog:

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

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 facebook#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

Pull Request resolved: facebook#55817

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)

Reviewed By: cortinico

Differential Revision: D95034757

Pulled By: cipolleschi

fbshipit-source-id: 1db7aecbd1d13fd52ad45aadff1bd0c1a6168236

Author: tux2nicolae

packages/react-native/Libraries/AppDelegate/RCTDefaultReactNativeFactoryDelegate.mm

@@ -45,6 +45,10 @@ - (JSRuntimeFactoryRef)createJSRuntimeFactory
 {
 #if USE_THIRD_PARTY_JSC != 1
   return jsrt_create_hermes_factory();
+#else
+  [NSException raise:@"JSRuntimeFactory"
+              format:@"createJSRuntimeFactory must be overridden when using third-party JSC"];
+  return nil;
 #endif
 }
 

packages/react-native/scripts/react_native_pods.rb

@@ -78,7 +78,7 @@ def use_react_native! (
   react_native_path = Pod::Config.instance.installation_root.join(path)
   prefix = react_native_path.relative_path_from(Pod::Config.instance.installation_root)
 
-  hermes_enabled= true
+  hermes_enabled= !use_third_party_jsc()
   # Set the app_path as env variable so the podspecs can access it.
   ENV['APP_PATH'] = app_path
   ENV['REACT_NATIVE_PATH'] = react_native_path.to_s