Avoid redundant pending-indentation writes (#26774)
## Summary
The lexer checks `pending_indentation` for every token. `Option::take`
writes `None` even on the common path where no indentation is pending,
so we now read the field first and clear it only when a value is
present. This avoids a redundant write in the hot lexer loop while
preserving the existing indentation behavior.
This PR is stacked on #26765 so that CodSpeed can measure the
pending-indentation optimization independently.
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Co-authored-by: Micha Reiser <micha@reiser.io>
