NPC config in MDX should accept LLM model input

[harshithmullapudi]

Problem

When authoring NPC config in MDX, the config currently does not accept an LLM model parameter. This makes it impossible for content authors to explicitly choose which LLM/model should be used for a given NPC.

Request

Add support for specifying an LLM model (e.g., model name/identifier) as part of the NPC config when authored in MDX — whether via a config block, frontmatter, or component props (whatever is consistent with the existing MDX API).

Acceptance criteria

1. MDX syntax supports specifying model for NPC config (e.g., model: "gpt-4.1" / claude-3-7-sonnet / etc.).
2. Config validation is updated to include the model field (including type/allowed format as appropriate).
3. Default behavior remains unchanged when model is omitted (existing MDX content continues to work).
4. Docs/examples are updated to show the new model field in NPC config within MDX.