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_request.go

@@ -0,0 +1,5 @@
+package responses
+
+func ConvertOpenAIResponsesRequestToGemini(modelName string, rawJSON []byte, stream bool) []byte {
+	return nil
+}