[Bug Fix] [RB] Fix path callback Swift name

[Kyle-Ye]

Summary

Fix the RenderBox path callback flags Swift importer name by using the valid nested type name RBPath.CallbacksFlags instead of the unsupported RBPath.Callbacks.Flags spelling.

The source header and packaged RenderBox framework header copies are updated together so consumers no longer see the invalid Swift name warning.

Root Cause

The Swift importer accepts the RBPath.Callbacks record mapping, but rejects the deeper RBPath.Callbacks.Flags record name and reports an invalid base identifier warning.

Validation

• Typechecked the packaged RenderBox framework header through swiftc -typecheck -import-objc-header and verified RBPath.CallbacksFlags imports without the warning.
• Built OpenSwiftUI with local dependencies enabled:
  OPENSWIFTUI_USE_LOCAL_DEPS=1 OPENRENDERBOX_USE_LOCAL_DEPS=1 OPENATTRIBUTEGRAPH_USE_LOCAL_DEPS=1 swift build --target OpenSwiftUI
• Confirmed the local-dependency build log has no Callbacks.Flags or invalid swift_name warning matches.