fix: preserve pages when moving document blocks

[kaluli123123]

Summary

• avoid deleting a sub-page when its document block is moved or rearranged
• preserve pages whose view parent already changed during a sidebar move
• only treat sub_page blocks as references, so other blocks sharing view_id do not suppress deletion
• cover nested references and non-sub-page nodes with the same attribute

Closes #7588

Validation

• flutter test --no-pub test/unit_test/document/sub_page_transaction_handler_test.dart — 1 test passed
• flutter analyze --no-pub . — no issues found
• Dart formatting and git diff --check pass

Validated with Flutter 3.27.4 on macOS arm64.

[sourcery-ai]

Reviewer's Guide

Updates sub-page transaction handling to distinguish actual deletions from block moves and sidebar reparenting, preserving views that remain referenced or have already changed parents. Adds a unit test for recursive nested-reference detection.

Sequence diagram for preserving sub-pages during block removal

sequenceDiagram
    participant Editor as EditorState
    participant Handler as SubPageTransactionHandler
    participant Backend as ViewBackendService

    Editor->>Handler: _subPageDeleted(context, editorState, node, parentViewId)
    Handler->>Editor: documentContainsSubPageReference(root, viewId)
    alt another block references viewId
        Editor-->>Handler: true
        Handler-->>Editor: preserve view
    else no remaining reference
        Handler->>Backend: getView(viewId)
        Backend-->>Handler: view
        alt view.parentViewId != parentViewId
            Handler-->>Editor: preserve reparented view
        else same parent
            Handler->>Backend: deleteView(viewId: viewId)
            Backend-->>Handler: result
        end
    end

Loading

File-Level Changes

Change	Details	Files
Prevent deletion of pages that are still referenced or whose hierarchy changed during block moves.	Recursively scan the document tree for remaining sub-page references before deleting a page. Pass the removed block’s original parent view ID into deletion handling. Fetch the referenced view and delete it only when it still belongs to the removed block’s parent. Skip deletion when the view is missing, has no ID, or has already been reparented.	frontend/appflowy_flutter/lib/plugins/document/presentation/editor_plugins/sub_page/sub_page_transaction_handler.dart
Add coverage for nested sub-page reference detection.	Verify references are found beneath nested nodes. Verify unrelated view IDs are not reported as references.	frontend/appflowy_flutter/test/unit_test/document/sub_page_transaction_handler_test.dart

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#7588	Prevent a page containing a document block from being deleted or moved to the trash when that block is moved through the sidebar or rearranged in the document.	✅
#7588	Preserve the referenced page when another document block still references it, including references nested inside layouts such as columns.	✅
#7588	Add regression coverage for detecting nested sub-page references.	✅

Possibly linked issues

• [Bug] Moving a page that has an existing document block will send it to the trash can instead #7588: The PR directly fixes the reported deletion behavior by preserving referenced pages during document block moves and undo operations.

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