Fix macOS system window tabs sync after merging windows

[zjarlin]

Summary

• sync GPUI system window tab groups from AppKit tabbedWindows after macOS finishes mergeAllWindows:
• keep the existing in-memory merge behavior as a fallback when the platform does not report tabbed windows
• reuse the macOS merge callback path so menu-driven merges and programmatic merges update the same state

Fixes #49724

Testing

• cargo fmt --all
• cargo check -p gpui
• cargo check -p platform_title_bar
• cargo check -p gpui_macos --features runtime_shaders

cargo check -p gpui_macos without runtime_shaders was blocked locally because this machine only has Command Line Tools selected and xcrun could not find metal/xcodebuild. The runtime_shaders check validates the Rust changes without offline Metal shader compilation.

[cla-bot]

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