set_model validation errors now carry code invalid_request

What

set_model is a control request, a message an app built on the Claude Code SDK sends to change the model during a session. When it rejects bad input, its error now includes code: "invalid_request" next to the existing error message. This covers:

• a model value that is not a string ("set_model: model must be a string")
• an empty system_prompt ("set_model: system_prompt must be a non-empty string when present")

Before, these errors had only ok and error.

Why

Apps that talk to Claude Code can check a fixed code to spot bad input, instead of matching the wording of the error message, which may change.