Upgrade breakdown enum mismatch from WARNING to ERROR

[MaxGhenis]

Summary

Parameters with keys not in the breakdown variable's possible values now raise ValueError instead of silently logging a warning. This prevents data loss when parameter YAML files use keys that don't match the breakdown enum.

For example, SNAP utility allowance parameters with breakdown: [state_code] but using snap_utility_region keys (AK_C, NY_NYC, etc.) were silently dropped — only a WARNING was logged. This caused real data loss in PolicyEngine/policyengine-us#7726.

Changes

• homogenize_parameters.py: Collect all extra children (keys not in breakdown enum) and raise ValueError with a clear message listing the offending keys
• test_nesting.py: Add tests for both the error case (extra keys → ValueError) and the allowed case (partial coverage → no error)

Design

• yaml_keys - enum_values non-empty → ERROR (unknown keys that would be silently dropped)
• enum_values - yaml_keys non-empty → OK (partial coverage is intentional, e.g. state_code breakdown omitting armed forces codes)

Closes #444

Test plan

• Existing test_parameter_homogenization still passes (valid breakdown)
• New test_breakdown_mismatch_raises_error passes (extra key raises ValueError)
• New test_breakdown_partial_coverage_is_ok passes (missing enum values allowed)
• CI passes on all Python versions

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