fix: move caret to previous line on arrow-up inside code blocks

[BetterAndBetterII]

Summary

• Arrow-up inside a multi-line code block used geometry-based vertical movement, which includes the language-picker header and jumped to the previous block.
• Register a newline-based arrowUpInCodeBlock handler before arrowUpToTitle so Up moves to the previous line (same column, clamped) while the first line still falls through to the default handler.

Fixes #8936

Test plan

• flutter test test/unit_test/document/shortcuts/code_block_arrow_command_test.dart (7 cases)
• Manual: in a multi-line code block, Up from line 2+ stays in the block on the previous line; Up from line 1 leaves the block

Summary by Sourcery

Fix Up-arrow navigation in multi-line code blocks so the caret moves between code lines without skipping to the previous block.

Bug Fixes:

• Keep the caret within multi-line code blocks when pressing Up, moving to the previous line at the same column and clamping to shorter lines.
• Allow Up from the first code-block line to continue to the default behavior and leave the block.

Tests:

• Add unit coverage for code-block vertical movement, column clamping, boundary behavior, and ignored cases.

[CLAassistant]

All committers have signed the CLA.

[sourcery-ai]

Reviewer's Guide

Introduces a prioritized, newline-based Up-arrow handler for code blocks, keeping the caret within the block on lines after the first while preserving existing fallback behavior at the first line and in other contexts.

Sequence diagram for code block Arrow-Up handling

sequenceDiagram
    participant User
    participant Shortcuts as Command shortcuts
    participant Handler as arrowUpInCodeBlock
    participant Editor as EditorState
    participant Default as Default arrow-up handler

    User->>Shortcuts: Arrow Up
    Shortcuts->>Handler: _arrowUpInCodeBlockHandler(editorState)
    Handler->>Editor: getNodeAtPath(selection.end.path)
    alt Collapsed caret in a code block after the first line
        Handler->>Editor: selection = Selection.collapsed(previous line offset)
        Handler-->>User: KeyEventResult.handled
    else First line or non-code-block context
        Handler-->>Shortcuts: KeyEventResult.ignored
        Shortcuts->>Default: Handle Arrow Up
    end

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File-Level Changes

Change	Details	Files
Adds a dedicated Up-arrow command that performs newline-based vertical navigation within code blocks.	Validates collapsed selections located in code-block nodes. Calculates the previous line and preserves the caret column, clamping to the shorter line. Returns ignored on the first line or for unsupported selection/node states so existing handlers can continue.	frontend/appflowy_flutter/lib/plugins/document/presentation/editor_plugins/code_block/code_block_arrow_command.dart
Registers the code-block navigation command ahead of broader Up-arrow handlers.	Places the new command before title and standard arrow-up shortcuts to prevent geometry-based movement across the code-block header or into the previous node.	frontend/appflowy_flutter/lib/plugins/document/presentation/editor_plugins/shortcuts/command_shortcuts.dart
Adds unit coverage for code-block Up-arrow behavior and fallback cases.	Tests same-column movement, line transitions, column clamping, empty lines, first-line fallback, non-code-block selections, and expanded selections.	frontend/appflowy_flutter/test/unit_test/document/shortcuts/code_block_arrow_command_test.dart

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#8936	Ensure pressing the Up arrow from a line after the first line inside a multi-line code block moves the caret to the preceding line within the same code block, preserving the column where possible and clamping it for shorter lines.	✅
#8936	Allow pressing the Up arrow from the first line of a code block to retain the existing behavior of moving to the preceding block or node.	✅
#8936	Add automated coverage for code-block Up-arrow behavior, including line movement, column clamping, first-line fallback, and ignored cases.	✅

Possibly linked issues

• [Bug] Code block cursor issue #8936: The PR directly fixes the reported cursor jump by moving Up to the previous code-block line.

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