Fix batch result format corruption when using cached prepared statements

Copy ResultFormats slice before passing to SendQueryStatement, which
stores it for later use. The extended query builder reuses the backing
array on the next loop iteration, corrupting format codes stored for
earlier queries in the batch.

Fixes the bug exposed by the test in jackc#2524.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jackc

conn.go

@@ -1235,7 +1235,11 @@ func (c *Conn) sendBatchExtendedWithDescription(ctx context.Context, b *Batch, d
 		if bi.sd.Name == "" {
 			pipeline.SendQueryParams(bi.sd.SQL, c.eqb.ParamValues, bi.sd.ParamOIDs, c.eqb.ParamFormats, c.eqb.ResultFormats)
 		} else {
-			pipeline.SendQueryStatement(bi.sd, c.eqb.ParamValues, c.eqb.ParamFormats, c.eqb.ResultFormats)
+			// Copy ResultFormats because SendQueryStatement stores the slice for later use, and eqb.Build reuses the
+			// backing array on the next iteration.
+			resultFormats := make([]int16, len(c.eqb.ResultFormats))
+			copy(resultFormats, c.eqb.ResultFormats)
+			pipeline.SendQueryStatement(bi.sd, c.eqb.ParamValues, c.eqb.ParamFormats, resultFormats)
 		}
 	}