Match MySQL mixed-type result and comparison semantics for IF(), CASE, and IFNULL() #519

Summary

Implement MySQL-compatible mixed-type result conversion and comparison semantics for IF(), CASE, and IFNULL().

This is a pre-existing compatibility gap, separate from the IF() condition-evaluation bug in #517. Keep #518 focused on that bug. Build this follow-up on the comparison work in #508.

Reproduction and rationale

SELECT IF(1, 2, '3') = '2';

MySQL returns 1. The raw SQLite translation returns 0:

SELECT CASE WHEN 1 THEN 2 ELSE '3' END = '2';

SQLite preserves the selected branch's integer storage class. MySQL applies result-type rules across the branches. Related mixed-type conversion gaps affect CASE and IFNULL().

An unconditional text cast can fix this example but break numeric comparisons. The solution must account for both result-type rules and the surrounding comparison semantics.

Required changes

• Determine MySQL result-type and comparison rules for mixed numeric, string, and NULL values in IF(), CASE, and IFNULL().
• Extend the translator and comparison handling from Fix DATE_FORMAT formatting and numeric comparisons #508 to reproduce those rules consistently.
• Preserve lazy evaluation of unselected branches and fallback expressions.
• Add regression tests against MySQL reference results for string comparisons, numeric comparisons, NULLs, nested expressions, and column-derived values.
• Avoid an unconditional text cast as the general solution.

Affected areas

• packages/mysql-on-sqlite/src/sqlite/class-wp-mysql-on-sqlite.php: translate_runtime_function_call(), expression translation, and comparison handling.
• packages/mysql-on-sqlite/tests/WP_MySQL_On_SQLite_Tests.php: regression coverage for IF(), CASE, and IFNULL().

Acceptance criteria

• SELECT IF(1, 2, '3') = '2' returns 1, matching MySQL.
• Mixed-type string and numeric comparison cases for all three constructs match MySQL reference results.
• NULL handling and lazy evaluation remain correct.
• Existing IF() condition-evaluation and comparison tests continue to pass.

Context

Requested by @JanJakes.

• Originating PR: Fix IF() condition evaluation #518
• Review discussion: Fix IF() condition evaluation #518 (comment)
• Related comparison work: Fix DATE_FORMAT formatting and numeric comparisons #508