feat: add insertText command

[fendent]

Summary

Adds a new insertText command that inserts plain text into the input without replacing existing rich content (images, mentions, etc).

Accepts optional start and end position parameters (visible indices, skipping ZWSPs). When omitted or set to -1, inserts at the current selection/cursor position.

setValue replaces the entire content, and setImage replaces the selection with an image, but there's no way to programmatically insert plain text at a position without destroying rich content.

This is intentionally plain-text-only and complements #409 (insertValue), which targets rich/HTML content insertion.

inputRef.current.insertText(' ');           // insert at cursor
inputRef.current.insertText('hello', 5);    // insert at position 5
inputRef.current.insertText('hi', 2, 5);    // replace range 2-5

Test Plan

1. Insert an inline image via setImage
2. Call insertText(' ') — a space should appear after the image without destroying it
3. Call insertText('hello', 0) — text should be inserted at position 0
4. Call insertText('x', 2, 5) — should replace characters 2-5 with 'x'
5. Verify cursor moves to the end of inserted text in all cases

Compatibility

OS	Implemented
iOS	✅
Android	✅

Checklist

• E2E tests are passing
• Required E2E tests have been added (if applicable)