fix(converters): emit schema-conformant OSI documents (drop non-spec root dialects/vendors)

[andreybavt]

Problem

Output from the dbt→OSI converter fails OSI's own validator (validation/validate.py)
against the published core schema:

[Schema] (root): Additional properties are not allowed ('dialects' was unexpected)

Because the CLI writes via to_osi_yaml(), every dbt→OSI conversion produces a
non-conformant document.

Root cause

OSIDocument (python/src/osi/models.py) declared optional root-level
dialects/vendors fields that aren't in core-spec/osi-schema.json, whose root is
additionalProperties: false (only version + semantic_model). The dbt converter
set dialects=[self._dialect], so dialects was emitted at the document root.

Fix

• Remove the non-spec dialects/vendors fields from OSIDocument.
• Stop the dbt converter from populating a document-root dialect.

Per-expression dialect tagging (OSIExpression.dialects) is unchanged and remains the
schema-valid home for dialects, so no information is lost. Dialect selection still
flows end-to-end; the two affected converter tests now assert it on the per-expression path.

Regression guard

Adds converters/dbt/tests/test_schema_conformance.py, which converts a representative
manifest and validates the emitted document (YAML and JSON, for ANSI_SQL and
SNOWFLAKE) against core-spec/osi-schema.json, reusing validation/validate.py.
CI now fails if a converter emits a non-conformant document root again.

Testing

• validate.py on converter output: fails before ('dialects' was unexpected),
  passes after.
• Full dbt converter test suite green.

Out of scope

Whether OSI should support a document-level (default) dialect is the open discussion in
#52 (one dialect per document) and #16 (default dialect at dataset level). This PR takes
no position and makes no schema change - it only aligns the reference model and
converter with the schema as published today.

[andreybavt]

@khush-bhatia , let me know if this first PR follows the community guidelines. Happy to take bigger scope after this first one lands

[khush-bhatia]

Thanks for the fix.