Rollup merge of #83348 - osa1:issue83344, r=jackh726

Rollup merge of #83348 - osa1:issue83344, r=jackh726

format macro argument parsing fix

When the character next to `{}` is "shifted" (when mapping a byte index
in the format string to span) we should avoid shifting the span end
index, so first map the index of `}` to span, then bump the span,
instead of first mapping the next byte index to a span (which causes
bumping the end span too much).

Regression test added.

Fixes #83344

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