Advertise JSON Schema 2020-12 dialect on emitted tool schemas

[koic]

Motivation and Context

MCP 2025-11-25 (SEP-1613) establishes JSON Schema 2020-12 as the default dialect for MCP schema definitions.
modelcontextprotocol/modelcontextprotocol#1613

The Ruby SDK was emitting tool input and output schemas with no $schema field, leaving the dialect implicit and inconsistent with the TypeScript SDK (packages/core/src/util/standardSchema.ts, which emits with target: 'draft-2020-12') and the Python SDK (which preserves $schema in its types and conformance fixtures).

Tool::Schema#to_h now includes "$schema": "https://json-schema.org/draft/2020-12/schema" unless the user supplied a different $schema value, so clients see the canonical dialect URI on the wire.

Runtime metaschema and argument validation remain on draft-04 because the json-schema gem does not yet support 2020-12. Switching the runtime validator is left as a separate change because it requires either replacing the gem or waiting for upstream 2020-12 support.

How Has This Been Tested?

Updated Tool::Schema#to_h expectations across input_schema_test, output_schema_test, tool_test, and the tools/list server tests to reflect the new $schema field. Added focused regression tests:

• to_h preserves a user-supplied $schema URI rather than overwriting it (covered for both symbol-keyed input and the string-keyed Hash form that goes through the initializer's symbolize_names normalization)
• validate_arguments / validate_result continue to work when the user supplies a 2020-12 $schema URI on the schema (the validator strips the unknown top-level dialect URI before invoking the draft-04 validator, instead of raising JSON::Schema::SchemaError)
• tools/list JSON-RPC response includes the 2020-12 $schema on both inputSchema and outputSchema (transport-boundary assertion)

bundle exec rake test and bundle exec rake rubocop both pass.

Breaking Changes

Additive change. Tolerant consumers of the tools/list payload are unaffected. Consumers that strictly assert hash equality on the emitted inputSchema / outputSchema need to account for the new $schema key, but no schema semantics change.

Schemas using 2020-12-only keywords (e.g. prefixItems, nested $schema on subschemas) are still rejected by the draft-04 metaschema validator, as before. Lifting that restriction depends on adopting a 2020-12-capable validator and is out of scope for this change.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context