fix: preserve cached table list shape

[memleakd]

Description

Preserves the documented list<string> return shape of BaseConnection::listTables() when returning cached table names.

When a table is dropped, the cached table-name array may become sparse. Returning array_values() from the cached path keeps the result as a proper list, matching the method PHPDoc and the fresh-query path.

This was split out from #10169 after review because it is a small database metadata fix with its own regression coverage.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide