refactor: share compiler IN clause fragments

[kholdrex]

Summary

Refactors the compiler string builder so top-level and related-subquery IN filters share the same fragment construction path. This keeps parameter lookup, array validation, placeholder numbering, and bind metadata consistent while reducing duplicate compiler logic.

Changes

• Extracts shared IN clause fragment construction into a private compiler helper.
• Reuses the helper for both normal filters and related subquery filters.
• Adds regression coverage for placeholder and bind ordering around top-level and subquery IN filters.

Test Plan

• Compiler specs passed.
• Full RSpec suite passed.
• RuboCop passed.