MDEV-39942: use actual_rec_per_key in loose scan sj optimization

[bsrikanth-mariadb]

While performing loose scan semi-join optimization in
Loose_scan_opt::check_ref_access_part1() [opt_subselect.h],
the filtered value in the explain plan was in-consistent for a table
with index when same data was inserted using "insert into t1 values()...",
and "insert into t1 select * from t2". This is noticed when
rec_per_key[] was used to get the number of records during optimization.

However, when actual_rec_per_key() is used to get the number of records,
we don't notice this discrepancy.

This PR changes the rec_per_key[] usage to actual_rec_per_key() in the
method Loose_scan_opt::check_ref_access_part1() [opt_subselect.h]

[gemini-code-assist]

Code Review

This pull request updates Loose_scan_opt in sql/opt_subselect.h to call actual_rec_per_key() instead of directly accessing the rec_per_key array. The reviewer pointed out a potential precision loss issue because the returned floating-point value is assigned to a ulong variable (rpc), which can lead to inaccurate optimizer estimations. A refactoring was suggested to use a double type and avoid assignment inside the conditional statement.

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