Add drag-and-drop tab merging into split panes

[d-init-d]

Summary

• Add an attach-as-pane drag/drop path so a dragged tab can merge into an existing tab as a split pane instead of always opening as a separate tab.
• Add tab-header split intent detection plus a content overlay with directional drop targets and preview states for up, right, down, left, and automatic center drops.
• Preserve existing pane/session content during the move and restore the focused pane when replaying multi-pane tab layouts.

Implementation Checklist

• Extend drag/drop metadata and remoting payloads to carry split intent and split direction.
• Detect split-intent drops on the center region of an existing tab while preserving normal tab reordering/move behavior.
• Add a dedicated content overlay for directional split previews so terminal file/text drag-drop stays isolated from the new tab-drop flow.
• Rebuild dragged tabs as split-pane actions that reuse existing ContentId state instead of starting fresh sessions.
• Restore the focused pane when attaching multi-pane dragged tabs into an existing tab.
• Clear preview state when drag/drop completes, leaves the target surface, or creates a new window.

Verification

• git diff --check
• Branch pushed to fork: d-init-d:feature/tab-drag-split-pane
• Full local build is currently blocked in this environment by missing prerequisites required by the repo:
  • Windows SDK 10.0.22621.0
  • .NET Framework 4.7.2 Developer Pack / targeting pack
  • Microsoft.NET.Sdk

Notes

• Refs Drag & Drop Tabs to create a Pane #4587
• Opened as a draft first because this is a feature-level change and the local environment here is missing the repo's required build prerequisites.

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[zadjii-msft]

Can you please install the SDK and actually build this locally? It seems weird to have us review code that you're not even sure compiles cleanly, let alone works the way you expect.

When you do have it running, can you share a screen recording of it in action?