fix(gateway): aggregate all text chunks in non-streaming Gemini responses

Previously, collectGeminiSSE() only returned the last chunk received
from the upstream streaming response when converting to non-streaming.
This caused incomplete responses where only the final text fragment
was returned to clients.

For example, a request asking to "count from 1 to 10" would only
return "\n" (the last chunk) instead of "1\n2\n3\n...\n10\n".

This was especially problematic for JSON structured output where
the opening brace "{" from the first chunk was lost, resulting
in invalid JSON like: colors": ["red", "blue"]}

The fix:
- Collect all text parts from each SSE chunk into a slice
- Merge all collected text parts into the final response
- Reuse the same pattern as handleGeminiStreamToNonStreaming
  in antigravity_gateway_service.go

Fixes: non-streaming responses returning incomplete text
Fixes: structured output (JSON schema) returning invalid JSON

backend/internal/service/gemini_messages_compat_service.go

@@ -1972,6 +1972,7 @@ func collectGeminiSSE(body io.Reader, isOAuth bool) (map[string]any, *ClaudeUsag
 
 	var last map[string]any
 	var lastWithParts map[string]any
+	var collectedTextParts []string // Collect all text parts for aggregation
 	usage := &ClaudeUsage{}
 
 	for {
@@ -1983,7 +1984,7 @@ func collectGeminiSSE(body io.Reader, isOAuth bool) (map[string]any, *ClaudeUsag
 				switch payload {
 				case "", "[DONE]":
 					if payload == "[DONE]" {
-						return pickGeminiCollectResult(last, lastWithParts), usage, nil
+						return mergeCollectedTextParts(pickGeminiCollectResult(last, lastWithParts), collectedTextParts), usage, nil
 					}
 				default:
 					var parsed map[string]any
@@ -2002,6 +2003,12 @@ func collectGeminiSSE(body io.Reader, isOAuth bool) (map[string]any, *ClaudeUsag
 						}
 						if parts := extractGeminiParts(parsed); len(parts) > 0 {
 							lastWithParts = parsed
+							// Collect text from each part for aggregation
+							for _, part := range parts {
+								if text, ok := part["text"].(string); ok && text != "" {
+									collectedTextParts = append(collectedTextParts, text)
+								}
+							}
 						}
 					}
 				}
@@ -2016,7 +2023,7 @@ func collectGeminiSSE(body io.Reader, isOAuth bool) (map[string]any, *ClaudeUsag
 		}
 	}
 
-	return pickGeminiCollectResult(last, lastWithParts), usage, nil
+	return mergeCollectedTextParts(pickGeminiCollectResult(last, lastWithParts), collectedTextParts), usage, nil
 }
 
 func pickGeminiCollectResult(last map[string]any, lastWithParts map[string]any) map[string]any {
@@ -2029,6 +2036,83 @@ func pickGeminiCollectResult(last map[string]any, lastWithParts map[string]any)
 	return map[string]any{}
 }
 
+// mergeCollectedTextParts merges all collected text chunks into the final response.
+// This fixes the issue where non-streaming responses only returned the last chunk
+// instead of the complete aggregated text.
+func mergeCollectedTextParts(response map[string]any, textParts []string) map[string]any {
+	if len(textParts) == 0 {
+		return response
+	}
+
+	// Join all text parts
+	mergedText := strings.Join(textParts, "")
+
+	// Deep copy response
+	result := make(map[string]any)
+	for k, v := range response {
+		result[k] = v
+	}
+
+	// Get or create candidates
+	candidates, ok := result["candidates"].([]any)
+	if !ok || len(candidates) == 0 {
+		candidates = []any{map[string]any{}}
+	}
+
+	// Get first candidate
+	candidate, ok := candidates[0].(map[string]any)
+	if !ok {
+		candidate = make(map[string]any)
+		candidates[0] = candidate
+	}
+
+	// Get or create content
+	content, ok := candidate["content"].(map[string]any)
+	if !ok {
+		content = map[string]any{"role": "model"}
+		candidate["content"] = content
+	}
+
+	// Get existing parts
+	existingParts, ok := content["parts"].([]any)
+	if !ok {
+		existingParts = []any{}
+	}
+
+	// Find and update first text part, or create new one
+	newParts := make([]any, 0, len(existingParts)+1)
+	textUpdated := false
+
+	for _, p := range existingParts {
+		pm, ok := p.(map[string]any)
+		if !ok {
+			newParts = append(newParts, p)
+			continue
+		}
+		if _, hasText := pm["text"]; hasText && !textUpdated {
+			// Replace with merged text
+			newPart := make(map[string]any)
+			for k, v := range pm {
+				newPart[k] = v
+			}
+			newPart["text"] = mergedText
+			newParts = append(newParts, newPart)
+			textUpdated = true
+		} else {
+			newParts = append(newParts, pm)
+		}
+	}
+
+	if !textUpdated {
+		newParts = append([]any{map[string]any{"text": mergedText}}, newParts...)
+	}
+
+	content["parts"] = newParts
+	result["candidates"] = candidates
+
+	return result
+}
+
 type geminiNativeStreamResult struct {
 	usage        *ClaudeUsage
 	firstTokenMs *int