Fix duplicated react/utils prefix in React-utils static header layout (#58759)

Summary:
`React-utils`'s `platformios` subspec inherits `header_dir = "react/utils"` from the parent spec, and maps its headers relative to `platform/ios` — a directory whose own layout already begins with `react/utils`. Outside `use_frameworks!` both layers are applied in sequence (`PodTarget#header_mappings`), so `FollyConvert.h` / `LowPriorityExecutor.h` end up one level too deep:

```
Pods/Headers/{Public,Private}/React-utils/react/utils/react/utils/*.h
```

and the documented include path can no longer be resolved. Since #58619 the `react/utils` umbrella is reached through `react/renderer/debug/flags.h` and `debugStringConvertibleUtils.h`, so the failure is not limited to direct consumers of `react/utils`: in the default static configuration any pod that transitively includes those headers stops compiling, e.g. in target `React-ImageManager`:

```
Pods/Headers/Public/React-utils/React/Utils.h:36:10: fatal error:
'react/utils/LowPriorityExecutor.h' file not found
```

Inside `use_frameworks!` the framework header copy path only takes the mapping directory into account (`pod_target_installer#add_header`; `link_headers` skips framework targets, which is why `Pods/Headers` is empty there), so `platform/ios` is correct for that configuration and only the static layout is broken.

The fix clears the inherited `header_dir` on that subspec instead of moving the mapping directory: the mapping already contributes the `react/utils` prefix, so the static layout becomes `Pods/Headers/Public/React-utils/react/utils/*.h` — the same place the module's other headers land — while the frameworks layout (`React_utils.framework/Headers/react/utils/*.h`) is unchanged, because `header_dir` does not participate in that path. `header_dir = ""` together with `header_mappings_dir = …` is also the convention the `*Umbrella` subspecs of this repo already use (`React-utils`, `React-Mapbuffer`, `React-FabricImage`, `ReactCommon`).

## Changelog:

[iOS] [FIXED] - React-utils: link `platform/ios` headers at `react/utils/…` in non-frameworks builds instead of a duplicated `react/utils/react/utils/…`, restoring `#include <react/utils/…>` and `<React/Utils.h>`

Pull Request resolved: #58759

Test Plan: RNTester using `pod install` or `USE_FRAMEWORKS=dynamic pod install` to build and can build successfully.

Reviewed By: javache

Differential Revision: D122536016

Pulled By: cipolleschi

fbshipit-source-id: e0bf2a0ecef6e8db713a276eb9a2f07b5f2daad0

Author: zhongwuzw

packages/react-native/ReactCommon/react/utils/React-utils.podspec

@@ -58,6 +58,7 @@ Pod::Spec.new do |s|
 
   s.subspec "platformios" do |ss|
     ss.source_files        = podspec_sources("platform/ios/**/*.{m,mm,cpp,h}", "platform/ios/**/*.h")
+    ss.header_dir          = ""
     ss.header_mappings_dir = "platform/ios"
   end