Add headers validation check to prevent crash (facebook#55749)

Summary:

Add defensive checks when processing custom headers to ensure:
1. Header keys are valid NSString instances before using them
2. Header values are successfully converted before adding to the request

This prevents potential crashes when invalid header data (non-string keys or values that fail conversion) is passed from JavaScript to the WebSocket module.

Changelog: [Internal]

Differential Revision: D94375533

Author: vzaidman

packages/react-native/React/CoreModules/RCTWebSocketModule.mm

@@ -96,8 +96,16 @@ - (void)invalidate
   // Load supplied headers
   if ([options.headers() isKindOfClass:NSDictionary.class]) {
     NSDictionary *headers = (NSDictionary *)options.headers();
-    [headers enumerateKeysAndObjectsUsingBlock:^(NSString *key, id value, BOOL *stop) {
-      [request addValue:[RCTConvert NSString:value] forHTTPHeaderField:key];
+    [headers enumerateKeysAndObjectsUsingBlock:^(id key, id value, BOOL *stop) {
+      if (![key isKindOfClass:[NSString class]]) {
+        RCTAssert(NO, @"RCTWebSocketModule: Invalid header key type - expected NSString");
+        return;
+      }
+      NSString *headerValue = [RCTConvert NSString:value];
+      if (headerValue == nil) {
+        headerValue = @"";
+      }
+      [request addValue:headerValue forHTTPHeaderField:key];
     }];
   }