Fail on columns the contract does not declare, when the schema says so

[markhm]

Fixes #1722.

Problem

A contract cannot say that a file must have no columns besides the ones it declares. An extra column passes unnoticed. ODCS v3.2.0 has no field for this.

What this changes

A schema opts in with a custom property, named after the same idea in JSON Schema:

schema:
  - name: orders
    customProperties:
      - property: additionalProperties
        value: false

• The schema then gets a check, "Check that orders has no fields the contract does not declare" (type model_no_additional_fields, dimension conformity). It fails on columns the schema does not declare and names them: Fields not in the contract: note, with additional_fields in the diagnostics.
• Names compare case-insensitively, like the existing field presence check.
• It reads only the column names, never the rows, so it also runs with --metadata-only. On a CSV file it reads the {model}__raw__ view; elsewhere the table itself.
• It works on every server read through ibis: CSV, JSON and Parquet files, and database tables. SAP HANA has its own engine and is not covered.
• The value may be the boolean false or the text "false".
• Without the custom property nothing changes.

Tests

tests/test_test_additional_fields.py: 9 tests, covering an extra column, exactly the declared columns, case, no custom property, text values, a JSON file, --metadata-only, and a DuckDB database table. uv run pytest: all pass. ruff: clean.

Docs

schema.md: a row in the checks table and a short section on the setting. CHANGELOG entry under Added.

Note

This PR and #1724 touch a comment in check_spec.py and the same bullet in schema.md, so the one merged second needs a small rebase. I'll do that.

• Tests pass (uv run pytest)
• Code formatted (uv run ruff check --fix && uv run ruff format)
• Docs updated (if relevant)
• CHANGELOG.md entry added

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