feat(providers): add extra_body config to inject custom fields into request body

Allow configuring provider-specific fields like reasoning_split for minimax via
the model config's extra_body map. These fields are merged into the request
body last, giving them precedence over default values.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/frontend/src/api/models.ts

@@ -17,6 +17,7 @@ export interface ModelInfo {
   max_tokens_field?: string
   request_timeout?: number
   thinking_level?: string
+  extra_body?: Record<string, any>
   // Meta
   configured: boolean
   is_default: boolean