Fix MySQL DELETE using CONCAT_WS preventing index usage

[hashwnath]

Summary

Fixes #5711

• Override _replace_by_key in MySQLEngineAdapter to use a JOIN-based DELETE for composite keys instead of CONCAT_WS('__SQLMESH_DELIM__', ...) which prevents MySQL/MariaDB from using indexes
• Single-key DELETEs continue to use the existing efficient IN-based approach since indexes work fine for single columns
• Add tests verifying both composite key (JOIN-based) and single key (IN-based) DELETE behavior

Problem

When using INCREMENTAL_BY_UNIQUE_KEY with MySQL/MariaDB and composite keys, SQLMesh generates:

DELETE FROM `target_table`
WHERE CONCAT_WS('__SQLMESH_DELIM__', `col1`, `col2`)
IN (SELECT CONCAT_WS('__SQLMESH_DELIM__', `col1`, `col2`) FROM `temp_table`)

MySQL cannot use indexes on CONCAT_WS() expressions, causing full table scans that lead to multi-hour query times and connection timeouts on larger tables.

Fix

For composite keys, generate a JOIN-based DELETE that allows MySQL to use existing indexes:

DELETE `_target`
FROM `target_table` AS `_target`
INNER JOIN `temp_table` AS `_temp`
ON `_target`.`col1` = `_temp`.`col1`
AND `_target`.`col2` = `_temp`.`col2`

This changes the EXPLAIN plan from type=ALL (full scan) to type=ref (index lookup), reducing query times from hours to seconds.

Test plan

• New test test_replace_by_key_composite_uses_join_delete verifies JOIN-based DELETE for composite keys
• New test test_replace_by_key_single_key_uses_in verifies single-key DELETEs still use the IN-based approach
• Existing MySQL adapter tests pass
• Existing mixin tests pass
• Existing base engine adapter tests pass