Unify filter condition builders and handler code paths

[seapagan]

Summary

The query filtering code currently has two parallel implementations for condition construction:

• handler path: _handle_equality, _handle_in, _handle_like, _handle_comparison
• builder path: _build_sql_condition, _build_in_condition, _build_like_condition

This duplicates operator/type validation and SQL fragment rules, which increases drift risk over time.

Goal

Refactor filter handling so operator/type validation and SQL fragment construction are centralized in the _build_* condition functions, while preserving existing behavior.

Scope

• Refactor handler methods to delegate to builder functions
• Keep existing self.filters contract intact (or migrate in a fully consistent way)
• Preserve exact:
  • exception types and messages
  • SQL clause semantics
  • parameter ordering
  • NULL and COLLATE NOCASE behavior
• Update/add tests to prevent behavior drift

Non-goals

• No new filter operators
• No public API changes
• No query planner/performance rewrites outside this refactor

Acceptance Criteria

• Existing filter tests pass unchanged (or with only necessary structural updates)
• Added regression tests for:
  • __eq, __isnull, __notnull
  • __in, __not_in
  • __like, __startswith, __endswith, __contains, case-insensitive variants
  • comparison operators
  • type validation error parity
• Output SQL fragments/params remain behaviorally equivalent across code paths