Allow $ref in Tool::Schema for protocol version 2025-11-25

`Tool::Schema` rejected `$ref` with an `ArgumentError`.
Earlier MCP spec versions (2024-11-05 through 2025-06-18) did not define
a JSON Schema dialect or reference the `$ref` keyword. `inputSchema` was described
only as "JSON Schema defining expected parameters" with no further constraints:

- https://modelcontextprotocol.io/specification/2024-11-05/server/tools
- https://modelcontextprotocol.io/specification/2025-06-18/server/tools

The MCP spec 2025-11-25 introduced a "JSON Schema Usage" section that adopts
JSON Schema 2020-12 as the default dialect. Since `$ref` is a core keyword in 2020-12,
it is now allowed for protocol version 2025-11-25 and later.

- https://modelcontextprotocol.io/specification/2025-11-25/basic#json-schema-usage
- https://modelcontextprotocol.io/specification/2025-11-25/server/tools
- https://json-schema.org/draft/2020-12/release-notes

For backward compatibility with older protocol versions (2025-06-18 and earlier),
`Server#validate!` continues to raise `ArgumentError` when a tool input schema contains `$ref`.
This preserves the existing behavior for servers targeting those spec versions.

lib/mcp/tool/schema.rb

@@ -31,10 +31,6 @@ module MCP
         case schema
         when Hash
           schema.each_with_object({}) do |(key, value), result|
-            if key.casecmp?("$ref")
-              raise ArgumentError, "Invalid JSON Schema: $ref is not allowed in tool schemas"
-            end
-
             result[yield(key)] = deep_transform_keys(value, &block)
           end
         when Array