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/codex/openai/responses/codex_openai-responses_response.go

@@ -30,8 +30,8 @@ func ConvertCodexResponseToOpenAIResponses(ctx context.Context, modelName string
 // from a non-streaming OpenAI Chat Completions response.
 func ConvertCodexResponseToOpenAIResponsesNonStream(_ context.Context, modelName string, originalRequestRawJSON, requestRawJSON, rawJSON []byte, _ *any) string {
 	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)
 	dataTag := []byte("data:")
 	for scanner.Scan() {
 		line := scanner.Bytes()