feat(registry/runtime): add Gemini 2.5 model and increase buffer sizes

- Added new "Gemini 2.5 Flash Image Preview" model definition, with enhanced image generation capabilities.
- Increased scanner buffer size to 20,971,520 bytes across executors and translators to handle larger payloads.

internal/translator/claude/gemini/claude_gemini_response.go

@@ -331,8 +331,8 @@ func ConvertClaudeResponseToGeminiNonStream(_ context.Context, modelName string,
 	streamingEvents := make([][]byte, 0)
 
 	scanner := bufio.NewScanner(bytes.NewReader(rawJSON))
-	buffer := make([]byte, 10240*1024)
-	scanner.Buffer(buffer, 10240*1024)
+	buffer := make([]byte, 20_971_520)
+	scanner.Buffer(buffer, 20_971_520)
 	for scanner.Scan() {
 		line := scanner.Bytes()
 		// log.Debug(string(line))