Make snippet editor search persistent and safe #515

Problem

The CodeMirror search dialog opened by Ctrl+F / Cmd+F is non-persistent. Pressing Enter closes the dialog and returns focus to the snippet editor, including when there are no matches. Users may then accidentally type, paste, or press Enter in their code.

Forum report: https://wordpress.org/support/topic/issues-caused-by-the-built-in-search-behavior/

Current behaviour

Code Snippets already loads CodeMirror's persistent findPersistent command on Alt+F. Ctrl+F / Cmd+F still use CodeMirror's default non-persistent find command.

Acceptance criteria

• Map Ctrl+F and Cmd+F to CodeMirror's existing findPersistent command.
• Keep focus in the search input after Enter.
• Escape or an explicit close action returns focus to the code editor.
• Make the search field sufficiently wide for typical code queries.
• Show an accessible, visible no-results state.
• Verify Default, Vim, Emacs, and Sublime Text keymaps do not regress.
• Verify Tab inserts or indents immediately under the Default keymap.

Scope

Reuse CodeMirror's search API. Do not build a separate search engine unless its API cannot meet these requirements.