feat: add image generation for OpenAI DALL-E and Gemini Imagen

New `/v1/images/generations` endpoint proxies DALL-E 2/3 (OpenAI)
and Imagen 3 (Gemini). Same auth/logging as chat completions.

- Add ImageGenerationRequest/Response models
- Extend Provider interface with ImageGeneration()
- OpenAI: forward to /v1/images/generations
- Gemini: call /v1beta/models/{model}:predict, map OpenAI params
- Circuit breaker wraps image gen like chat completions
- Model routing: dall-e* -> openai, imagen*/gemini* -> gemini
- Unsupported providers (deepseek/moonshot/grok/ollama) return error
- Fix pre-existing CachedContentTokenCount bug in StreamGemini

internal/providers/circuit_breaker.go

@@ -54,3 +54,13 @@ func (cbp *CircuitBreakerProvider) ChatCompletionStream(ctx context.Context, req
 	// Future: Implement a way to track stream failures in the circuit breaker.
 	return cbp.provider.ChatCompletionStream(ctx, req)
 }
+
+func (cbp *CircuitBreakerProvider) ImageGeneration(ctx context.Context, req *models.ImageGenerationRequest) (*models.ImageGenerationResponse, error) {
+	result, err := cbp.cb.Execute(func() (interface{}, error) {
+		return cbp.provider.ImageGeneration(ctx, req)
+	})
+	if err != nil {
+		return nil, err
+	}
+	return result.(*models.ImageGenerationResponse), nil
+}