fix(driver): validate FROM clause in COUNT query generation

[cofin]

Summary

Fixes COUNT query generation to properly validate FROM clause existence before attempting to create COUNT(*) queries. Addresses upstream bug report where select_with_total raised confusing error for SQL with only ORDER BY clause.

The Problem

select_with_total() raised "Cannot create COUNT query from empty SQL expression" when SELECT statement lacked FROM clause (e.g., "SELECT * ORDER BY id"). The root cause was attempting to use expr.args.get("from") without validating it wasn't None, which would pass None to sqlglot's .from_() method.

The Solution

Added explicit validation using sqlglot AST inspection to check expr.args.get("from") exists before attempting COUNT generation. Provides clear, actionable error message: "SELECT statement missing FROM clause. COUNT queries require a FROM clause to determine which table to count rows from."

Key Changes

• Validation: Check if not expr.args.get("from") before building COUNT query
• Clear error message: Explains why FROM clause is required for COUNT queries
• Comprehensive tests: 12 unit tests covering edge cases (ORDER BY only, WHERE only, empty SELECT, etc.)
• Backwards compatible: No behavior change for valid SELECT...FROM queries

Test Coverage

• ✅ 12 new unit tests in test_count_query_edge_cases.py
• ✅ Tests missing FROM clause scenarios (ORDER BY only, WHERE only, SELECT *)
• ✅ Tests valid SELECT...FROM cases still work correctly
• ✅ Tests error message clarity and actionability
• ✅ All existing tests pass (4100+ tests with no regressions)

Example Error Messages

Before (confusing):

ImproperConfigurationError: Cannot create COUNT query from empty SQL expression

After (clear):

ImproperConfigurationError: Cannot create COUNT query: SELECT statement missing FROM clause. 
COUNT queries require a FROM clause to determine which table to count rows from.

Testing

# Run new tests
uv run pytest tests/unit/test_driver/test_count_query_edge_cases.py -v

# Run full suite
uv run pytest -n 2 --dist=loadgroup