Add OpenAI compatibility support and improve resource cleanup

- Introduced OpenAI compatibility configurations for external providers, enabling model alias routing via the OpenAI API format.
- Enhanced provider logic in `GetProviderName` to handle OpenAI aliases and added new helper functions for compatibility checks.
- Updated API handlers and client initialization to support OpenAI compatibility models.
- Improved resource cleanup across clients by closing response bodies and streams using deferred functions.

internal/constant/constant.go

@@ -1,9 +1,10 @@
 package constant
 
 const (
-	GEMINI    = "gemini"
-	GEMINICLI = "gemini-cli"
-	CODEX     = "codex"
-	CLAUDE    = "claude"
-	OPENAI    = "openai"
+	GEMINI               = "gemini"
+	GEMINICLI            = "gemini-cli"
+	CODEX                = "codex"
+	CLAUDE               = "claude"
+	OPENAI               = "openai"
+	OPENAI_COMPATIBILITY = "openai-compatibility"
 )