Scenario validator does not check literal option compatibility in 'when' expressions #118

Summary

Scenario validation checks expression syntax and unknown attribute names, but does not validate that compared string literals are valid categorical options for the referenced attribute.

This allows case/value mismatches (e.g., urban_rural == 'urban' when option is Urban) to pass validation and silently no-op at runtime.

Why This Matters

This is a major source of "looks valid but behavior is wrong" bugs. Incorrect conditions don’t throw hard errors during simulation; they just fail to match and flatten dynamics.

Current Behavior (Code)

In /Users/adithyasrinivasan/Projects/extropy/extropy/scenario/validator.py:

• syntax check: validate_expression_syntax(...)
• reference check: extract_names_from_expression(...) vs known attrs
• no check that literals used in comparisons are present in attribute option domains

By contrast, population semantic validation already has this concept via AST comparison extraction.

Proposed Fix

1. Add AST-based comparison extraction for scenario when clauses:

• seed_exposure.rules[].when
• timeline exposure rules timeline[].exposure_rules[].when (if present)
• spread.share_modifiers[].when

2. For each (attribute, compared_string_values) pair:

• if attribute is categorical with known options, require literal values to match one of those options
• invalid literals should be ERROR (not warning) because rule is effectively broken

3. Handle list membership checks (in [...]) and single comparisons (==, !=).

4. Add tests covering:

• exact match pass
• case mismatch fail
• nonexistent value fail
• non-categorical attributes skipped

Acceptance Criteria

• Invalid categorical literals in scenario conditions are caught at validation time.
• Common mismatch classes (case/style/legacy tokens) no longer survive to runtime.
• Validation message includes valid option set for quick fix.

Pipeline Impact

Reduces recursive debug loops by catching scenario-domain mismatches before sampling/simulation.