Fix nullability annotations in iOS networking headers (#59240)

Summary:
Importing `RCTMultipartDataTask.h` and `RCTHTTPRequestHandler.h` produces nullability-completeness warnings because the headers mix annotated return values with unannotated pointers and blocks.

Complete the annotations with `NS_ASSUME_NONNULL`, keeping optional configuration providers, interceptors, progress handlers and multipart callback payloads nullable. This matches the existing implementations, including clearing an interceptor with `nil` and reporting a failed multipart response without headers or content.

## Changelog:

[IOS] [FIXED] - Fix missing nullability annotations in networking headers.

Pull Request resolved: #59240

Test Plan:
- Compiled a translation unit importing both headers with the Xcode 27.1 iOS Simulator SDK, ARC, blocks and `-Werror`. Passed in Objective-C and Objective-C++ modes. It also exercises clearing all three providers/interceptors with `nil`, returning `nil` from each interceptor, nullable multipart callback payloads and an absent progress callback.
- The same header import on the original revision fails with nine `-Wnullability-completeness` errors.
- Compiled `RCTMultipartDataTask.m` with `-fsyntax-only -Werror=nullability-completeness` against the same SDK. Passed.
- Checked both changed headers with the repository's pinned clang-format 21.1.2 (`--dry-run --Werror`) and ran `git diff --check`. Passed.

A full RNTester build was not run for this header-only change.

Snapshot completion (local update):
- Added the four generated ReactApple API snapshots from the CI artifact for PR head `151876277233befdbe7c0d9818039aa08ad15d1c`; verified the artifact SHA-256 against GitHub metadata. Imported headers and all prior snapshots match that revision. No header implementation or dependency changes.
- The repository `validate_snapshots` comparison passes for all 12 generated views. `arc lint -a` reports no lint issues.
- Full local `buck2 run fbsource//xplat/js/react-native-github/scripts/cxx-api:api-snapshot` could not regenerate: the existing codegen environment lacks `tinyglobby`. The updated files are exact generated CI output, not hand-edited snapshots.

Reviewed By: christophpurrer

Differential Revision: D126472293

Pulled By: javache

fbshipit-source-id: 0c4f87e4d6f8c57f89f9e914d1c44a60802df1b6

Author: mrousavy

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

@@ -8,24 +8,28 @@
 #import <React/RCTInvalidating.h>
 #import <React/RCTURLRequestHandler.h>
 
+NS_ASSUME_NONNULL_BEGIN
+
 typedef NSURLSessionConfiguration *_Nullable (^NSURLSessionConfigurationProvider)(void);
 /**
  *  The block provided via this function will provide the NSURLSessionConfiguration for all HTTP requests made by the
  * app.
  */
-RCT_EXTERN void RCTSetCustomNSURLSessionConfigurationProvider(NSURLSessionConfigurationProvider /*provider*/);
+RCT_EXTERN void RCTSetCustomNSURLSessionConfigurationProvider(NSURLSessionConfigurationProvider _Nullable /*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*/);
+RCT_EXTERN void RCTSetCustomHTTPRequestInterceptor(RCTHTTPRequestInterceptor _Nullable /*interceptor*/);
 
 /**
  * This is the default RCTURLRequestHandler implementation for HTTP requests.
  */
 @interface RCTHTTPRequestHandler : NSObject <RCTURLRequestHandler, RCTInvalidating>
 
 @end
+
+NS_ASSUME_NONNULL_END

packages/react-native/React/Base/RCTMultipartDataTask.h

@@ -10,11 +10,13 @@
 #import <React/RCTDefines.h>
 #import <React/RCTMultipartStreamReader.h>
 
+NS_ASSUME_NONNULL_BEGIN
+
 typedef void (^RCTMultipartDataTaskCallback)(
     NSInteger statusCode,
-    NSDictionary *headers,
-    NSData *content,
-    NSError *error,
+    NSDictionary *_Nullable headers,
+    NSData *_Nullable content,
+    NSError *_Nullable error,
     BOOL done);
 
 typedef NSURLRequest *_Nullable (^RCTMultipartDataTaskRequestInterceptor)(NSURLRequest *request);
@@ -23,14 +25,17 @@ typedef NSURLRequest *_Nullable (^RCTMultipartDataTaskRequestInterceptor)(NSURLR
  * requests before they are sent. Return a modified request to override, or nil
  * to use the original request unchanged.
  */
-RCT_EXTERN void RCTSetCustomMultipartDataTaskRequestInterceptor(RCTMultipartDataTaskRequestInterceptor /*interceptor*/);
+RCT_EXTERN void RCTSetCustomMultipartDataTaskRequestInterceptor(
+    RCTMultipartDataTaskRequestInterceptor _Nullable /*interceptor*/);
 
 @interface RCTMultipartDataTask : NSObject
 
 - (instancetype)initWithURL:(NSURL *)url
                 partHandler:(RCTMultipartDataTaskCallback)partHandler
-            progressHandler:(RCTMultipartProgressCallback)progressHandler;
+            progressHandler:(nullable RCTMultipartProgressCallback)progressHandler;
 
 - (void)startTask;
 
 @end
+
+NS_ASSUME_NONNULL_END

scripts/cxx-api/api-snapshots/ReactAppleDebugCxx.api

@@ -1410,7 +1410,7 @@ interface RCTMountingManager : public NSObject {
 }
 
 interface RCTMultipartDataTask : public NSObject {
-  public virtual instancetype initWithURL:partHandler:progressHandler:(NSURL* url, RCTMultipartDataTaskCallback partHandler, RCTMultipartProgressCallback progressHandler);
+  public virtual instancetype initWithURL:partHandler:progressHandler:(NSURL* url, RCTMultipartDataTaskCallback partHandler, _Nullable RCTMultipartProgressCallback progressHandler);
   public virtual void startTask();
 }
 

scripts/cxx-api/api-snapshots/ReactAppleFrameworksCxx.api

@@ -1453,7 +1453,7 @@ interface RCTMountingManager : public NSObject {
 }
 
 interface RCTMultipartDataTask : public NSObject {
-  public virtual instancetype initWithURL:partHandler:progressHandler:(NSURL* url, RCTMultipartDataTaskCallback partHandler, RCTMultipartProgressCallback progressHandler);
+  public virtual instancetype initWithURL:partHandler:progressHandler:(NSURL* url, RCTMultipartDataTaskCallback partHandler, _Nullable RCTMultipartProgressCallback progressHandler);
   public virtual void startTask();
 }
 

scripts/cxx-api/api-snapshots/ReactAppleNewarchCxx.api

@@ -1410,7 +1410,7 @@ interface RCTMountingManager : public NSObject {
 }
 
 interface RCTMultipartDataTask : public NSObject {
-  public virtual instancetype initWithURL:partHandler:progressHandler:(NSURL* url, RCTMultipartDataTaskCallback partHandler, RCTMultipartProgressCallback progressHandler);
+  public virtual instancetype initWithURL:partHandler:progressHandler:(NSURL* url, RCTMultipartDataTaskCallback partHandler, _Nullable RCTMultipartProgressCallback progressHandler);
   public virtual void startTask();
 }
 

scripts/cxx-api/api-snapshots/ReactAppleReleaseCxx.api

@@ -1410,7 +1410,7 @@ interface RCTMountingManager : public NSObject {
 }
 
 interface RCTMultipartDataTask : public NSObject {
-  public virtual instancetype initWithURL:partHandler:progressHandler:(NSURL* url, RCTMultipartDataTaskCallback partHandler, RCTMultipartProgressCallback progressHandler);
+  public virtual instancetype initWithURL:partHandler:progressHandler:(NSURL* url, RCTMultipartDataTaskCallback partHandler, _Nullable RCTMultipartProgressCallback progressHandler);
   public virtual void startTask();
 }