`@remotion/layout-utils`: Fix fitTextOnNLines overflow for long words (#7359)

fillTextBox now rejects wrapping when the trimmed word alone exceeds maxBoxWidth, uses <= for line width, and preserves full Word styling when measuring. Adds Vitest browser tests and a Remotion studio testbed mirroring web-renderer.

Co-authored-by: Cursor <cursoragent@cursor.com>