Fix: Mock provider raises UnknownAttributeError when response_format is passed

[aoki-ryusei]

Summary

Fix ActiveModel::UnknownAttributeError raised when response_format is passed to the Mock provider.

Steps to Reproduce

class MockAgent < ApplicationAgent
  generate_with :mock

  # @return [ActiveAgent::Generation]
  def ask
    prompt(
      message: params[:message],
      response_format: :json_object
    )
  end
end

MockAgent.with(message: "What is ActiveAgent?").ask.generate_now

unknown attribute 'response_format' for ActiveAgent::Providers::Mock::Request. (ActiveModel::UnknownAttributeError)
      raise UnknownAttributeError.new(self, name)
      ^^^^^

Problem

The Mock provider documentation states that response_format is "accepted but not validated or enforced", but in practice passing it caused a crash.

The root cause was that Mock::Request had a hardcoded getter method but no attribute :response_format declaration.
When BaseProvider cast the context hash into the request object, ActiveModel tried to call response_format= as a
setter, which didn't exist, and raised the error.

# Before: hardcoded getter only, no setter
def response_format
  { type: "text" }
end

Fix

Add attribute :response_format to Mock::Request and remove the hardcoded getter. The attribute now accepts any value passed in, while mock behavior remains unchanged — no schema validation or structured output is enforced.

How to Verify

Run the reproduction steps above and confirm no error is raised.