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f267aed0 - [ty] Fix panic for annotation pointing at leading whitespace (#23458)

[ty] Fix panic for annotation pointing at leading whitespace (#23458) ## Summary When a diagnostic annotation pointed at indentation (e.g., an "unexpected indentation" error), `whitespace_left` could exceed `span_left`, causing a subtraction overflow in `Margin::compute`. Closes https://github.com/astral-sh/ty/issues/836.