Forward host:didInitializeRuntime: callback to user delegate (#57438)

Summary:
`RCTReactNativeFactory` acts as `RCTHost`'s delegate but did not implement `host:didInitializeRuntime:`, causing the optional callback to never reach the user's `RCTReactNativeFactoryDelegate`.

Add the missing forwarding method following the same pattern as `hostDidStart:` so the runtime reference is properly delivered to the user delegate.

Fixes #57349.

## Changelog:

[IOS] [FIXED] - Forward `host:didInitializeRuntime:` callback to user delegate in `RCTReactNativeFactory`

Pull Request resolved: #57438

Test Plan:
1. Implement `host:didInitializeRuntime:` in a custom `RCTReactNativeFactoryDelegate` subclass (e.g. RNTester's `AppDelegate`)
2. Build and launch the app
3. Verify the callback is invoked and the `jsi::Runtime` reference is received
4. Confirm `hostDidStart:` continues to work as before

NOTE: No automated test. This mirrors the existing hostDidStart: forwarding in the same RCTReactNativeFactory, which is also untested. host:didInitializeRuntime: is declared on RCTHostDelegate and inherited by RCTReactNativeFactoryDelegate under optional, and the respondsToSelector: guard makes it a no-op for delegates that don't implement it, so there's no logic to regress. No RCTReactNativeFactory delegate-forwarding test harness exists, so a test would be net-new iOS XCTest scaffolding disproportionate to a one-line forward.

Reviewed By: cortinico

Differential Revision: D113653288

Pulled By: fabriziocucci

fbshipit-source-id: 082bb340cd9d61bc30a10c8a7730eca14f69088e

Author: zhongwuzw

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

@@ -212,6 +212,13 @@ - (void)hostDidStart:(RCTHost *)host
   }
 }
 
+- (void)host:(RCTHost *)host didInitializeRuntime:(facebook::jsi::Runtime &)runtime
+{
+  if ([_delegate respondsToSelector:@selector(host:didInitializeRuntime:)]) {
+    [_delegate host:host didInitializeRuntime:runtime];
+  }
+}
+
 - (NSArray<NSString *> *)unstableModulesRequiringMainQueueSetup
 {
 #if RN_DISABLE_OSS_PLUGIN_HEADER