Use symbolize_names: true in JSON.parse calls

Replace custom `deep_transform_keys` in `Tool::Schema` and
string key access in `StreamableHTTPTransport` with
the `symbolize_names: true` option of `JSON.parse`.

This aligns `StreamableHTTPTransport` with `StdioTransport`,
which already uses `symbolize_names: true`.

lib/mcp/tool/schema.rb

@@ -8,7 +8,7 @@ module MCP
       attr_reader :schema
 
       def initialize(schema = {})
-        @schema = deep_transform_keys(JSON.parse(JSON.dump(schema)), &:to_sym)
+        @schema = JSON.parse(JSON.dump(schema), symbolize_names: true)
         @schema[:type] ||= "object"
         validate_schema!
       end
@@ -27,19 +27,6 @@ module MCP
         JSON::Validator.fully_validate(to_h, data)
       end
 
-      def deep_transform_keys(schema, &block)
-        case schema
-        when Hash
-          schema.each_with_object({}) do |(key, value), result|
-            result[yield(key)] = deep_transform_keys(value, &block)
-          end
-        when Array
-          schema.map { |e| deep_transform_keys(e, &block) }
-        else
-          schema
-        end
-      end
-
       def validate_schema!
         schema = to_h
         gem_path = File.realpath(Gem.loaded_specs["json-schema"].full_gem_path)