Refactor translator packages for OpenAI Chat Completions

- Renamed `openai` packages to `chat_completions` across translator modules.
- Introduced `openai_responses_handlers` with handlers for `/v1/models` and OpenAI-compatible chat completions endpoints.
- Updated constants and registry identifiers for OpenAI response type.
- Simplified request/response conversions and added detailed retry/error handling.
- Added `golang.org/x/crypto` for additional cryptographic functions.

internal/translator/gemini/openai/responses/gemini_openai-responses_response.go

@@ -0,0 +1,11 @@
+package responses
+
+import "context"
+
+func ConvertGeminiResponseToOpenAIResponses(_ context.Context, modelName string, rawJSON []byte, param *any) []string {
+	return nil
+}
+
+func ConvertGeminiResponseToOpenAIResponsesNonStream(_ context.Context, _ string, rawJSON []byte, _ *any) string {
+	return ""
+}