test: add 101 SQL syntax coverage tests for parser gaps

Closes gaps in type casting, join variants, CASE expressions, regex
functions, and underrepresented dialects (Snowflake, MySQL, Spark).

New test files:
- test_join_types.py (22 tests): INNER, LEFT, RIGHT, FULL OUTER, CROSS,
  self-join, multi-join, subquery join, ON clause lineage, 3 dialects
- test_type_casting.py (24 tests): CAST, ::, SAFE_CAST, TRY_CAST,
  cast in expressions, implicit conversions, 3 dialects
- test_case_expressions.py (16 tests): CASE WHEN, searched CASE, nested,
  aggregates, COALESCE, IF, IFF, NULLIF
- test_regex_functions.py (13 tests): REGEXP_EXTRACT, REGEXP_REPLACE,
  REGEXP_CONTAINS, RLIKE, PostgreSQL ~, 3 dialects
- test_dialect_coverage.py (25 tests): Snowflake FLATTEN/semi-structured,
  MySQL GROUP_CONCAT/IF/backticks, Spark EXPLODE/COLLECT_LIST/TRANSFORM,
  cross-dialect equivalence

Known gaps documented via xfail (2):
- MySQL INSERT ... ON DUPLICATE KEY UPDATE
- Spark CREATE TABLE USING delta

Author: mingjerli