Fix semantic UNKNOWN error

[ruizmaa]

Closes #80

Two unhandled crashes in the semantic analyser were raising UNKNOWN errors instead of producing a valid result.

1. Binary.validate_types in operators/base.py raised Exception("Mixed type promotion requires a result dataframe") when one operand was Mixed but no result dataframe existed (semantic-only mode, Scalar + ScalarSet path). Fixed by returning the operator's fixed return type directly when result_dataframe is None.

2. visitExprWithSelection in ast/constructor.py hardcoded ctx_list[3] as the body expression. When the optional [WHERE expression] block is present the body shifts to a higher index, causing visit(None) → NotImplementedError. Fixed by using ctx_list[-1].

Result

4 operations fixed.

operation_vid	code	release	expression
10474	v22841_a-1	4.0	{tB_05.01, c0070} in { [eba_CT:x212], [eba_CT:x213] }
10475	v22861_a-3	4.0	{tB_05.01, c0080} in { [eba_GA:AL], [eba_GA:AT], [eba_GA:BE], ... }
10477	v22874_a-1	4.0	{tB_05.01, c0070} in { [eba_CT:x212], [eba_CT:x213] }
19007	v12213_m	4.2	with {tR_04.00.a, c*, default: 0, interval: true} [where qPYB = [eba_qIA:qx2090]]: {r0100} + {r0060} >= 9000000 * {r0040} ...

This are the failures after the fix: test_data_failures.csv

Checklist

• Code quality checks pass (ruff format, ruff check, mypy)
• Tests pass (pytest) with 100% branch coverage (coverage report --fail-under=100)
• Documentation updated (if applicable)

Impact / Risk

• Breaking changes? (public API / CLI / REST endpoints / Django models)
• Database schema or migration concerns?
• Notes for release/changelog?

Notes