fix(validation): report schema errors for invalid YAML

[mattfaltyn]

Summary

JSON Schema validation already detects invalid YAML shapes, but the CLI continued into semantic checks that assume schema-valid mappings and lists. Those checks could raise AttributeError before the existing reporter displayed the schema errors.

This change:

• reports the schema errors for empty, sequence, scalar, and malformed nested YAML;
• runs semantic checks only after schema validation succeeds; and
• adds focused CLI regression coverage using the Python standard library.

The specification, CLI arguments, and dependency set are unchanged.

Related Issues

Closes #270

Testing

Focused validation

uv run --with 'jsonschema>=4.26.0' --with 'pyyaml>=6.0.3' \
  --with 'sqlglot>=30.12.0' python -m unittest validation.test_validate -v
uv run validation/validate.py /dev/null --schema core-spec/osi-schema.json
uv run validation/validate.py examples/tpcds_semantic_model.yaml \
  --schema core-spec/osi-schema.json
python -m py_compile validation/validate.py validation/test_validate.py
git diff --check

Results:

• The regression test passes all four input cases.
• Empty YAML exits 1 with the expected schema error and no traceback.
• The valid TPC-DS example passes.
• Compilation, Ruff, and diff checks pass.

Repository-wide validation

• All Python binding and converter suites pass except the Omni baseline noted below.
• CLI go build, go vet, and go test pass.
• Polaris and Salesforce Maven verify builds pass.
• The Omni suite has two pre-existing TPC-DS fixture failures, reproduced unchanged on the PR base (8c410f9); this PR does not modify Omni or the shared example.
• No current GitHub Actions workflow is path-matched to validation/**, so this PR has no remote checks.

Checklist

Validation

• New validation cases are covered by tests

Documentation

• Existing documentation was reviewed; no update is needed because the validator interface and documented contract are unchanged

Tests

• All tests relevant to this change pass locally
• New functionality is covered by tests

Compliance

• ASF license headers are present on all new source files
• No third-party dependencies are added