Agent API: preset renamed to effort

Perplexity renamed the Agent API's `preset` parameter to `effort`, a naming change that keeps the same models, prompts, tools, limits and pricing. `preset` is still accepted as a deprecated alias, and responses return both fields, so existing integrations keep working.

• Version pinning and frozen configurations continue to work as before.
• The Python SDK 0.46.0 and TypeScript SDK 0.41.0 add the `effort` parameter.