fix: ensure PR summarizer uses template for JSON format instructions

## Problem
PR summarizer was passing raw PR content directly to Gemini without
wrapping it in the prompt template, causing the AI to echo back the
PR metadata as JSON instead of generating a proper summary.

This resulted in error:
"422 Validation Failed - missing_field: title"

## Solution
- Modified GeneratePRSummary to call generatePRPrompt() to wrap PR
  content in the template with JSON format instructions
- Added validation to ensure title is not empty before returning
- Added detailed error message showing AI response when title is missing

## Impact
- PR summarization now works correctly
- AI generates proper JSON with title, body, and labels
- Better error messages for debugging

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: thomas-vilte

internal/infrastructure/ai/gemini/pull_requests_summarizer_service.go

@@ -52,9 +52,12 @@ func NewGeminiPRSummarizer(ctx context.Context, cfg *config.Config, trans *i18n.
 	}, nil
 }
 
-func (gps *GeminiPRSummarizer) GeneratePRSummary(ctx context.Context, prompt string) (models.PRSummary, error) {
+func (gps *GeminiPRSummarizer) GeneratePRSummary(ctx context.Context, prContent string) (models.PRSummary, error) {
 	modelName := string(gps.config.AIConfig.Models[config.AIGemini])
 
+	// Envolver el contenido del PR en el template con instrucciones de formato JSON
+	prompt := gps.generatePRPrompt(prContent)
+
 	genConfig := &genai.GenerateContentConfig{
 		Temperature:      float32Ptr(0.3),
 		MaxOutputTokens:  int32(10000),
@@ -82,6 +85,16 @@ func (gps *GeminiPRSummarizer) GeneratePRSummary(ctx context.Context, prompt str
 		return models.PRSummary{}, fmt.Errorf("error al parsear JSON de PR: %w", err)
 	}
 
+	// Validar que el título no esté vacío
+	if strings.TrimSpace(jsonSummary.Title) == "" {
+		respLen := len(responseText)
+		preview := responseText
+		if respLen > 500 {
+			preview = responseText[:500] + "..."
+		}
+		return models.PRSummary{}, fmt.Errorf("la IA no generó un título para el PR. Respuesta (longitud: %d): %s", respLen, preview)
+	}
+
 	return models.PRSummary{
 		Title:  jsonSummary.Title,
 		Body:   jsonSummary.Body,