Fix pinned preview drag and drop state

[wieslawsoltes]

Summary

• resolve pinned preview grip drags to the active tool across validation, preview, docking, and floating paths
• treat drops back onto the same pinned preview as a no-op
• preserve whole-dock dragging for regular tool docks
• add headless regressions for outer docking, self-drop, and outside-drop floating

Validation

• dotnet test tests/Dock.Avalonia.HeadlessTests/Dock.Avalonia.HeadlessTests.csproj -c Release --no-restore (617 passed)
• dotnet build Dock.slnx -c Release --no-restore

Closes #1053

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[chatgpt-codex-connector]

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