Description

Please describe your pull request.

This pull request fixes an issue where descending index columns (D) were not preserved when editing an index. When a column had a collation of D (meaning DESC), phpMyAdmin generated an index definition without specifying any direction.

Since MySQL and MariaDB default to ASC when no direction is specified, the
original DESC was effectively lost.

This bug was originally reported against the QA_5_2 branch:
Fixes #19749

The fix updates the SQL generation logic to explicitly map column collation
values:

• A → ASC
• D → DESC

ensuring the original index direction is preserved correctly.

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