Preserve code chunk selection when restoring visual editor focus

[kv9898]

Switching back to Positron from another application can move the visual editor's caret to the start of a code chunk. During focus restoration, focusing the outer ProseMirror editor can clear the browser selection before CodeMirror regains focus; CodeMirror then reads that DOM selection and resets its cursor.

Allow an extension to handle focus before the outer editor. When a text selection is wholly inside a CodeMirror chunk, restore that selection and focus CodeMirror directly. Other selections retain the existing focus path. The restoration delay and the separate editor-tab focus behavior are unchanged.

References #340 and posit-dev/positron#4248.

Validation:

• Manually reproduced the original failure in Positron and confirmed that the final implementation preserves the caret, selected code, and ordinary prose behavior across application switching.
• VS Code extension, visual editor, and browser fixture TypeScript checks pass.
• Browser fixture passes seven DOM restoration cases (start/middle/end, forward/backward selections, nested and empty chunks), prose focus, and three fallback-routing checks.
• Negative control: substituting the original outer-editor focus path fails the no-outer-focus assertion. A separate browser check confirms typing at the restored caret.

Run the browser fixture from the repository root:

node node_modules/vite/bin/vite.js packages/editor-codemirror/test

Open the printed local URL at /focus.html. No new dependencies are required. The fixture verifies restoration and routing; it does not reproduce the complete native application-switch failure on the original path. That failure and the final fix were verified manually in Positron.

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