Replace puts with injectable Logger instance

[Copilot]

All puts calls used for error logging were replaced with a proper Logger instance that is configurable globally and injectable per-component, eliminating the need for allow_any_instance_of monkey-patches in tests.

Changes

• Configuration — adds logger attr with default Logger.new($stdout), configurable via JSONRPC.configure
• Middleware — accepts logger: option; defaults to config.logger; passes logger down to Validator
• Validator — accepts logger: kwarg in constructor; defaults to config.logger for standalone use
• middleware_spec — replaces allow_any_instance_of with a let(:app) that injects an instance_double(Logger) directly into the middleware; replaces output(...).to_stdout with have_received(:error)

Usage

# Default: Logger.new($stdout), no changes needed
JSONRPC.configure do
  self.logger = Logger.new($stderr, level: :warn)
end

# Or inject per-middleware instance
use JSONRPC::Middleware, path: '/rpc', logger: Rails.logger

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