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>

pkg/config/defaults.go

@@ -339,6 +339,7 @@ func DefaultConfig() *Config {
 				ModelName: "MiniMax-M2.5",
 				Model:     "minimax/MiniMax-M2.5",
 				APIBase:   "https://api.minimaxi.com/v1",
+				ExtraBody:  map[string]any{"reasoning_split": true},
 			},
 
 			// LongCat - https://longcat.chat/platform