Keep converter outputs aligned with root schema

[LiangRuoShui]

Why

The published core JSON Schema only allows version and semantic_model at the document root and sets additionalProperties: false. dbt and Honeydew could still emit root-level advertisement fields (dialects / vendors), and the Python OSIDocument model accepted those fields, which allowed generated OSI documents to fail the official validator.

What changed

• Removed root-level dialects emission from the dbt MSI-to-OSI converter.
• Removed root-level vendors emission from the Honeydew-to-OSI converter.
• Made Honeydew reject root-level advertisement fields such as vendors and dialects on OSI input.
• Made the Python OSIDocument model reject schema-invalid root extra fields.
• Updated dbt snapshots to reflect schema-valid root output.

Tests added/updated

• Added dbt regression coverage asserting serialized OSI output omits root dialects and vendors while preserving expression-level dialects.
• Added Honeydew regression coverage asserting generated OSI output omits root advertisement fields.
• Added Honeydew input validation coverage asserting root vendors / dialects are rejected.
• Added Python model coverage asserting root dialects is rejected.

Verification

• uv run validation/validate.py examples/tpcds_semantic_model.yaml
• cd python && uv run --with pytest pytest -q
• cd converters/dbt && uv run pytest -q
• cd converters/honeydew && uv run pytest -q