BUG: read_csv low_memory mixed-dtype warning named the wrong columns (GH#67375)

[Parth1353]

• closes BUG: read_csv with usecols raises IndexError in low-memory mixed-dtype warning path #67375
• Tests added and passed
• All code checks passed
• Added an entry in the latest doc/source/whatsnew/v3.1.0.rst file
• AI was used to help create this PR

read_low_memory keys its chunks by field position in the source row, but
_concatenate_chunks used those positions to index the column-name list. The two only
line up when nothing was dropped, so the mixed-dtype DtypeWarning was wrong in three
ways — all from #58250, all shipped in 3.0.0, none present in 2.x:

1. usecols raised IndexError for any selected column not still at its original
   position (the reported bug). It reaches chunksize= and iterator= too, not just
   plain read_csv.
2. An index column silently named the wrong column. With index_col=0 a file whose
   mixed columns are c3, c50, c80 reported c4, c51, c81. No crash, no clue.
3. The number beside each name was a counter, not a position. ENH: DtypeWarning message enhancement #58174 asked to keep the
   existing Columns (15,19) positions and annotate them with names; ENH: DtypeWarning message enhancement #58250 numbered them
   with enumerate over the warned columns instead, so they became 0, 1, 2 … while still
   looking like the positions they replaced.

Fixed by mapping positions to labels the way read() renames these same keys: skip
leading_cols, then zip against orig_names — which usecols has already been applied to
and which the reader has already deduplicated. names is the wrong list; it drops index
columns.

A leading implicit index column has no name, so it stays a bare position rather than
borrowing the first data column's name. The mapping deliberately covers every parsed
position: a partial one turns a wrong label into a KeyError when that unnamed index
column is itself the mixed one.

Data was never affected — only the warning. Verified by cross-checking 28 option
combinations against engine="python" at both low_memory settings.

If the message change in (3) is unwanted here, it is easy to split out; (1) and (2) stand
on their own.

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AI disclosure: written with Claude Code (claude opus 5).