Update magnifier point at start (#19780)
fixes #19746
Summary of the issue:
Text caret follow would fail in some text editor (python console of nvda or notepad), if it would be the last character
Description of user facing changes:
text caret will now focus properly end character
Description of developer facing changes:
Description of development approach:
Update _get_pointAtStart now falls back to the previous character's position when the caret is at end-of-text and both _getBoundingRectFromOffset and _getPointFromOffset fail for the current offset.
