Add redbox subspec to React-debug pod (#56584)

Add a "redbox" subspec to React-debug.podspec with its own header_dir,
following the same pattern React-Fabric uses for its subdirectories.

Without this, static library builds (without USE_FRAMEWORKS) flatten all
headers into the top-level header_dir ("react/debug"), so imports like
`<react/debug/redbox/RedBoxErrorParser.h>` fail because the "redbox/"
subdirectory is lost.

Author: motiz88

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

@@ -25,11 +25,16 @@ Pod::Spec.new do |s|
   s.author                 = "Meta Platforms, Inc. and its affiliates"
   s.platforms              = min_supported_versions
   s.source                 = source
-  s.source_files           = podspec_sources("**/*.{cpp,h}", "**/*.h")
-  s.exclude_files          = "**/tests/**/*.{cpp,h}"
+  s.source_files           = podspec_sources("*.{cpp,h}", "*.h")
   s.header_dir             = "react/debug"
   s.pod_target_xcconfig    = { "CLANG_CXX_LANGUAGE_STANDARD" => rct_cxx_language_standard(),
                                "DEFINES_MODULE" => "YES" }
 
   resolve_use_frameworks(s, header_mappings_dir: "../..", module_name: "React_debug")
+
+  s.subspec "redbox" do |ss|
+    ss.source_files         = podspec_sources("redbox/*.{cpp,h}", "redbox/*.h")
+    ss.exclude_files        = "redbox/tests/**/*.{cpp,h}"
+    ss.header_dir           = "react/debug/redbox"
+  end
 end