fix(listen): warn when keywords is used with nova-3 model

[peteroyce]

Fixes #474

Adds a runtime console.warn() in WrappedListenV1Client.connect() when the caller passes both keywords and a nova-3 model (nova-3, nova-3-general, nova-3-medical). The Deepgram API silently rejects this combination with an HTTP 400, making it very hard to diagnose.

Before: The WebSocket upgrade fails with a bare HTTP 400 — no feedback to the developer.

After: A clear warning is printed before the connection attempt:

[Deepgram] The 'keywords' parameter is not supported with nova-3 models and will cause
the WebSocket connection to be rejected (HTTP 400). Use 'keyterm' instead.
See: https://developers.deepgram.com/docs/keyterm

Why CustomClient.ts: This file is not auto-generated by Fern and will survive SDK regenerations. The check is placed before the WebSocket is created so developers see the warning immediately.

No breaking changes — this is a warning only, existing behavior is unchanged.