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/constant/constant.go

@@ -1,10 +1,10 @@
 package constant
 
 const (
-	GEMINI               = "gemini"
-	GEMINICLI            = "gemini-cli"
-	CODEX                = "codex"
-	CLAUDE               = "claude"
-	OPENAI               = "openai"
-	OPENAI_COMPATIBILITY = "openai-compatibility"
+	GEMINI          = "gemini"
+	GEMINICLI       = "gemini-cli"
+	CODEX           = "codex"
+	CLAUDE          = "claude"
+	OPENAI          = "openai"
+	OPENAI_RESPONSE = "openai-response"
 )