ossie-dbt osi-to-msi crashes: PydanticSemanticManifest has no model_dump_json

`ossie-dbt osi-to-msi` fails on every input, including the repo's own example:

```
$ ossie-dbt osi-to-msi -i examples/tpcds_semantic_model.yaml -o out.json
File "converters/dbt/src/ossie_dbt/cli.py", line 78, in _cmd_osi_to_msi
output_path.write_text(result.output.model_dump_json(by_alias=True, exclude_none=True, indent=2))
AttributeError: 'PydanticSemanticManifest' object has no attribute 'model_dump_json'
```

`PydanticSemanticManifest` comes from `metricflow_semantic_interfaces` and subclasses `pydantic.v1.BaseModel`, so the serializer is `.json()`, not `.model_dump_json()`. Calling `.json(by_alias=True, exclude_none=True, indent=2)` on the same object produces valid output.

Reproduced on main at 88e0011. The conversion itself works — only the CLI write fails. No test covers the CLI, so CI passes.

Start in converters/dbt/src/ossie_dbt/cli.py at _cmd_osi_to_msi, then run the reported ossie-dbt osi-to-msi command with examples/tpcds_semantic_model.yaml. Verify that the CLI writes valid JSON to out.json and add coverage for this CLI path, since no existing test covers it.

Written by the indexing model from the issue text.