Adds reasoning enabled (#20)

Adds 'enabled' in PostOpenrouterReasoningRequest

Trying to fix this error
```
chat request failed: status_code: 400, url: https://openrouter.ai/api/v1/chat/completions body: {"error":{"message":"\"reasoning.max_tokens\" must be less than total tokens and greater than or equal to 1024","code":400,"metadata":{"provider_name":"Google"}}
```

Author: thihamin

chat.go

@@ -248,6 +248,9 @@ type PostOpenrouterReasoningRequest struct {
 
 	// Optional: Default is false. All models support this.
 	Exclude bool `json:"exclude"` // // Set to true to exclude reasoning tokens from response
+
+	// Or enable reasoning with the default parameters:
+	Enabled bool `json:"enabled"` // Default: inferred from `effort` or `max_tokens`
 }
 
 type StreamOptions struct {