Advertise JSON Schema 2020-12 dialect on emitted tool schemas

## Motivation and Context

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

lib/mcp/tool/schema.rb

@@ -5,6 +5,12 @@ require "json-schema"
 module MCP
   class Tool
     class Schema
+      # JSON Schema 2020-12 is the default dialect for MCP schema definitions
+      # per MCP 2025-11-25 (SEP-1613). Note: emission only — runtime validation
+      # is still performed against the JSON Schema draft-04 metaschema because
+      # the `json-schema` gem does not yet support 2020-12.
+      JSON_SCHEMA_2020_12_URI = "https://json-schema.org/draft/2020-12/schema"
+
       attr_reader :schema
 
       def initialize(schema = {})
@@ -18,17 +24,18 @@ module MCP
       end
 
       def to_h
-        @schema
+        return @schema if @schema.key?(:"$schema")
+
+        { "$schema": JSON_SCHEMA_2020_12_URI }.merge(@schema)
       end
 
       private
 
       def fully_validate(data)
-        JSON::Validator.fully_validate(to_h, data)
+        JSON::Validator.fully_validate(schema_for_validation, data)
       end
 
       def validate_schema!
-        schema = to_h
         gem_path = File.realpath(Gem.loaded_specs["json-schema"].full_gem_path)
         schema_reader = JSON::Schema::Reader.new(
           accept_uri: false,
@@ -38,11 +45,22 @@ module MCP
         # Converts metaschema to a file URI for cross-platform compatibility
         metaschema_uri = JSON::Util::URI.file_uri(metaschema_path.expand_path.cleanpath.to_s.tr("\\", "/"))
         metaschema = metaschema_uri.to_s
-        errors = JSON::Validator.fully_validate(metaschema, schema, schema_reader: schema_reader)
+        errors = JSON::Validator.fully_validate(metaschema, schema_for_validation, schema_reader: schema_reader)
         if errors.any?
           raise ArgumentError, "Invalid JSON Schema: #{errors.join(", ")}"
         end
       end
+
+      # The `json-schema` gem's draft-04 validator cannot resolve newer or unknown `$schema`
+      # dialect URIs. Strip the top-level `$schema` before validation so a dialect URI
+      # (whether SDK-injected by `to_h` or user-supplied) does not break the validator.
+      def schema_for_validation
+        return @schema unless @schema.key?(:"$schema")
+
+        copy = @schema.dup
+        copy.delete(:"$schema")
+        copy
+      end
     end
   end
 end