[codex] Improve code block insertion

[bendsp]

Summary

• make the fenced code block command insert an editable empty block on blank lines
• preserve indentation for empty fenced block snippets
• place the cursor inside the new fence and cover the behavior with unit tests

Validation

• pnpm lint
• pnpm typecheck
• pnpm test:unit

[Copilot]

Pull request overview

Improves the CodeMirror “fenced code block” toggle command so that when invoked on a blank/whitespace-only line with an empty selection, it inserts an empty fenced code block and places the cursor inside it (while preserving the line’s indentation).

Changes:

• Add a special-case path in toggleFencedCodeBlockCommand for empty selections on blank lines to insert an editable empty fenced block and move the cursor into it.
• Preserve leading indentation when inserting an empty fenced block snippet.
• Add unit tests covering empty-block insertion, cursor placement, and indentation preservation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/renderer/editor/codemirror/commands.ts	Adds blank-line empty-selection handling to insert an indented empty fenced block and position the cursor inside it.
tests/controllerShortcuts.test.ts	Adds unit tests validating the new empty fenced block insertion behavior and cursor position.

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