An AppServer client should be able to use any (`model_provider`, `model`) in the user's config. `NewConversationParams` already supported specifying the `model`, but this PR expands it to support `model_provider`, as well. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/openai/codex/pull/5793). * #5803 * __->__ #5793