Fix Omni pytest

[MonkeyCanCode]

Summary

This is sample fix for #260. There are two issues introduced by #113:

1. the new datatype field caused test_tpcds_export_matches_expected to fail due to Omni exporter lacks the logic to default is_time: true for temporal-typed fields whose explicit dimension role was unset
2. the test_osi_roundtrip_up_to_documented_normalizations fails due to non-native datatype property is dropped upon export but the test's normalization helper didn't remove it from the compared semantic model.

Related Issues

Checklist

Specification

• Spec changes are included in core-spec/ and follow the existing structure
• Spec changes have been discussed on the mailing list or in a linked issue
• Breaking changes to the spec are clearly called out in the summary

Ontology

• Ontology changes in ontology/ are consistent with spec changes
• New or modified terms are defined and documented

Converters

• Converter logic in converters/ is updated to reflect spec or ontology changes
• New converters include tests under the converter's test directory

Validation

• Validation rules in validation/ are updated if the spec changed
• New validation cases are covered by tests

Documentation

• docs/ is updated to reflect any user-facing changes
• New features or behaviors are documented with examples where appropriate
• CONTRIBUTING.md is updated if the contribution process changed

Examples

• examples/ are added or updated for any new spec constructs or converter support

Tests

• All existing tests pass (pytest / CI green)
• New functionality is covered by tests

Compliance

• ASF license headers are present on all new source files
• No third-party dependencies are added without PMC/IPMC approval