fix: WHERE filter pushdown silently corrupts OR/CASE queries

[jxom]

Problem

extract_raw_column_predicates used visit_expressions which recursively walks all sub-expressions, including children of OR and CASE nodes. This meant:

WHERE block_num = 1 OR block_num = 2

Would extract both block_num = 1 AND block_num = 2 as separate AND predicates in the CTE, silently changing query semantics from a disjunction to a conjunction — returning fewer rows than expected.

Fix

Replace visit_expressions with a manual walk (collect_and_conjuncts) that only recurses through top-level AND conjuncts. At OR, CASE, or any non-AND expression, we stop recursing. Only simple leaf predicates on raw columns are extracted.

Mixed expressions like a AND (b OR c) correctly push down only a.

Tests

• test_or_predicates_not_pushed_down — OR predicates are NOT pushed down
• test_case_predicates_not_pushed_down — CASE predicates are NOT pushed down
• test_simple_and_predicates_pushed_down — simple AND conjuncts still work
• test_mixed_and_or_only_pushes_safe_conjuncts — mixed AND/OR only pushes safe parts
• test_nested_or_inside_and_not_pushed_down — nested OR inside AND correctly skipped
• All 5 existing pushdown tests continue to pass