docs: default Ask AI to gpt-5.4-mini

Better grounding/instruction-following than gpt-4o-mini at docs-chat volumes;
still overridable via OPENAI_CHAT_MODEL.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ouiliame

apps/docs/app/api/chat/route.ts

@@ -9,7 +9,7 @@ export const runtime = 'nodejs'
 export const maxDuration = 30
 
 /** Model used for the Ask AI chat. Override with OPENAI_CHAT_MODEL in the environment. */
-const CHAT_MODEL = process.env.OPENAI_CHAT_MODEL || 'gpt-4o-mini'
+const CHAT_MODEL = process.env.OPENAI_CHAT_MODEL || 'gpt-5.4-mini'
 
 /** Max documentation chunks returned per search to ground an answer. */
 const SEARCH_LIMIT = 6