imf_query_dataset period validation
Reject malformed/reversed start_period and end_period before the upstream IMF call.
Fixed:
imf_query_datasetrejects malformedstart_period/end_periodand reversed ranges asValidationError(invalid_period_format,invalid_period_range) before calling IMF (#11)docs/design.md:key_dimension_mismatchandinvalid_sqlcorrected fromInvalidParamstoValidationError, matching source (#14)
83 tests pass; bun run devcheck clean.