chore(docs): add and refine package-level comments across modules

- Added detailed package-level comments to improve documentation coverage.
- Clarified parameter descriptions, return types, and functionality of exported methods across packages.
- Enhanced overall code readability and API documentation consistency.

internal/constant/constant.go

@@ -1,11 +1,27 @@
+// Package constant defines provider name constants used throughout the CLI Proxy API.
+// These constants identify different AI service providers and their variants,
+// ensuring consistent naming across the application.
 package constant
 
 const (
-	Gemini         = "gemini"
-	GeminiCLI      = "gemini-cli"
-	GeminiWeb      = "gemini-web"
-	Codex          = "codex"
-	Claude         = "claude"
-	OpenAI         = "openai"
+	// Gemini represents the Google Gemini provider identifier.
+	Gemini = "gemini"
+
+	// GeminiCLI represents the Google Gemini CLI provider identifier.
+	GeminiCLI = "gemini-cli"
+
+	// GeminiWeb represents the Google Gemini Web provider identifier.
+	GeminiWeb = "gemini-web"
+
+	// Codex represents the OpenAI Codex provider identifier.
+	Codex = "codex"
+
+	// Claude represents the Anthropic Claude provider identifier.
+	Claude = "claude"
+
+	// OpenAI represents the OpenAI provider identifier.
+	OpenAI = "openai"
+
+	// OpenaiResponse represents the OpenAI response format identifier.
 	OpenaiResponse = "openai-response"
 )