Add RCTHTTPRequestInterceptor for selective HTTP header injection (#55869)

Summary:
Pull Request resolved: #55869

## Summary:

Replace the blind application of dev headers on all HTTP requests with an
interceptor pattern. A custom `RCTHTTPRequestInterceptor` block can be
registered via `RCTSetCustomHTTPRequestInterceptor` to inspect and modify
HTTP requests before they are sent. The interceptor returns a modified request
to override, or nil to use the original request unchanged. This preserves the
shared NSURLSession configuration (including meta-auth mTLS) while allowing
selective per-request header injection.

- Declare `RCTHTTPRequestInterceptor` typedef and
  `RCTSetCustomHTTPRequestInterceptor` in RCTHTTPRequestHandler.h
- Implement the interceptor hook in RCTHTTPRequestHandler.mm
- Remove the previous blind `RCTDevSupportHttpHeaders` application

## Changelog:

[IOS][CHANGED] - Add RCTHTTPRequestInterceptor for selective HTTP request modification

## Test Plan:

Verified that the RCTHTTPRequestInterceptor block is called when registered,
and that returning nil leaves the original request unchanged. When no
interceptor is registered, behavior is unchanged from before.

## Facebook:
Applied from nest/mobile/apps/atod-sample/patches/react-native+0.83.1+006+atod-ios-http-handler.patch

Reviewed By: cortinico

Differential Revision: D95037920

fbshipit-source-id: ccd2c99c38eecb7fb5b074e7fce6f5abc3b6b3a1

Author: cipolleschi

packages/react-native/Libraries/Network/RCTHTTPRequestHandler.h

@@ -14,6 +14,15 @@ typedef NSURLSessionConfiguration * (^NSURLSessionConfigurationProvider)(void);
  * app.
  */
 RCT_EXTERN void RCTSetCustomNSURLSessionConfigurationProvider(NSURLSessionConfigurationProvider /*provider*/);
+
+typedef NSURLRequest *_Nullable (^RCTHTTPRequestInterceptor)(NSURLRequest *request);
+/**
+ * The block provided via this function can inspect/modify HTTP requests before
+ * they are sent. Return a modified request to override, or nil to use the
+ * original request unchanged.
+ */
+RCT_EXTERN void RCTSetCustomHTTPRequestInterceptor(RCTHTTPRequestInterceptor /*interceptor*/);
+
 /**
  * This is the default RCTURLRequestHandler implementation for HTTP requests.
  */

packages/react-native/Libraries/Network/RCTHTTPRequestHandler.mm

@@ -25,6 +25,13 @@ void RCTSetCustomNSURLSessionConfigurationProvider(NSURLSessionConfigurationProv
   urlSessionConfigurationProvider = provider;
 }
 
+static RCTHTTPRequestInterceptor httpRequestInterceptor;
+
+void RCTSetCustomHTTPRequestInterceptor(RCTHTTPRequestInterceptor interceptor)
+{
+  httpRequestInterceptor = interceptor;
+}
+
 @implementation RCTHTTPRequestHandler {
   NSMapTable *_delegates;
   NSURLSession *_session;
@@ -99,7 +106,14 @@ - (NSURLSessionDataTask *)sendRequest:(NSURLRequest *)request withDelegate:(id<R
                                            valueOptions:NSPointerFunctionsStrongMemory
                                                capacity:0];
   }
-  NSURLSessionDataTask *task = [_session dataTaskWithRequest:request];
+  NSURLRequest *finalRequest = request;
+  if (httpRequestInterceptor != nullptr) {
+    NSURLRequest *intercepted = httpRequestInterceptor(request);
+    if (intercepted != nil) {
+      finalRequest = intercepted;
+    }
+  }
+  NSURLSessionDataTask *task = [_session dataTaskWithRequest:finalRequest];
   [_delegates setObject:delegate forKey:task];
   [task resume];
   return task;