feat(sql): add CTE (WITH clause) support to SQL executor

[eddiethedean]

Summary

Add support for Common Table Expressions (CTEs) in SQL queries, enabling queries like:

WITH filtered_data AS (
    SELECT id, value FROM source_table WHERE value > 75
)
SELECT * FROM filtered_data

Changes

Parser (sparkless/session/sql/parser.py)

• Added WITH detection in _detect_query_type() (before UNION check, as CTE queries can contain UNION)
• Added routing for WITH in _parse_components()
• Implemented _parse_with_query() method that parses CTE definitions using balanced parenthesis counting
• Handles single CTE, multiple CTEs, and extracts main query

Executor (sparkless/session/sql/executor.py)

• Added WITH case in execute() method
• Implemented _execute_with() method that:
  • Executes each CTE query and stores results in temp views
  • Replaces CTE name references in main query with temp view names
  • Executes the main query (which can reference CTEs via temp views)
  • Cleans up temp views after execution

Features Supported

• ✅ Single CTE: WITH cte AS (...) SELECT * FROM cte
• ✅ Multiple CTEs: WITH cte1 AS (...), cte2 AS (...) SELECT * FROM cte2
• ✅ Chained CTEs (CTE referencing another CTE)
• ✅ CTEs with filtering, aggregation, and column selection
• ✅ Case-insensitive WITH keyword
• ✅ Proper cleanup of temp views after execution
• ✅ Works with ORDER BY, LIMIT, and other SQL clauses in main query

Test Plan

• 11 comprehensive unit tests covering all CTE scenarios
• Tests for: simple CTE, multiple CTEs, CTE chains, filtering, aggregation, case insensitivity, whitespace handling, parser detection, ORDER BY, LIMIT
• All tests pass

Implementation Details

The implementation uses balanced parenthesis counting to correctly parse CTE definitions, even when CTEs contain nested parentheses. Each CTE is executed independently and stored as a temporary view, which is then referenced in the main query. Temp views are automatically cleaned up after query execution.

Related

Fixes #347

This is an independent implementation, not based on PR #320.