Fix descenders cutoff in TextField (#26309)

* Include caret gaps in position calculation to avoid descender cutoff

* Calculate caret offset instead of exposing it

* Test that textfields scroll vertically by full line height

* Only add offset to end of caret to avoid going above 0.0

* Fix and test horizontal vs. vertical scrolling

* CR: docs and top caret offset logic

* Take max instead of assuming caretRect.top is always a full line down

* Always subtract caretOffset from caretTop since caretRect.top can be negative