fix(dbt): serialize osi-to-msi manifest with pydantic v1 .json()

[ntoxlut]

Summary

ossie-dbt osi-to-msi fails on every input, including the repository's own
examples/tpcds_semantic_model.yaml, with:

AttributeError: 'PydanticSemanticManifest' object has no attribute 'model_dump_json'

PydanticSemanticManifest (from metricflow_semantic_interfaces) subclasses
pydantic.v1.BaseModel, whose JSON serializer is .json(), not the pydantic v2
.model_dump_json(). The conversion itself works — only the CLI's write step
was calling a v2-only method on a v1 object.

Fix

• cli.py: use .json(by_alias=True, exclude_none=True, indent=2), which takes
  the same options on a pydantic v1 model and produces valid output.
• Add tests/test_cli.py — an end-to-end test that drives osi-to-msi and
  asserts a valid semantic manifest is written. No test covered the CLI before,
  which is why CI stayed green through the crash.

Verification

cd converters/dbt
uv run pytest          # 100 passed (was 99)
uv run ossie-dbt osi-to-msi -i ../../examples/tpcds_semantic_model.yaml -o /tmp/manifest.json
# -> Written to /tmp/manifest.json  (valid JSON)

The new test fails without the fix (AttributeError) and passes with it.

Related Issues

Fixes #296

Checklist

Specification

N/A — bug fix only, no spec change.

Ontology

N/A — no ontology change.

Converters

• New converters include tests under the converter's test directory

Validation

N/A — no validation change.

Documentation

N/A — no user-facing behavior change beyond the command no longer crashing.

Examples

N/A — no 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