Add OAuth support for Gemini CLI, Claude, Codex, and Qwen authentication

- Implemented new handlers (`RequestGeminiCLIToken`, `RequestAnthropicToken`, `RequestCodexToken`, `RequestQwenToken`) for initiating OAuth flows.
- Added endpoints for authorization URLs in management API.
- Extracted `GenerateRandomState` to a reusable utility in `misc` package.
- Refactored related logic in `openai_login.go` and `anthropic_login.go` for consistency.

Add endpoints for initiating OAuth flows in management API documentation

- Documented new endpoints for Anthropic, Codex, Gemini CLI, and Qwen login URLs.
- Provided request and response examples for each endpoint in both English and Chinese versions.

internal/misc/oauth.go

@@ -0,0 +1,21 @@
+package misc
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"fmt"
+)
+
+// GenerateRandomState generates a cryptographically secure random state parameter
+// for OAuth2 flows to prevent CSRF attacks.
+//
+// Returns:
+//   - string: A hexadecimal encoded random state string
+//   - error: An error if the random generation fails, nil otherwise
+func GenerateRandomState() (string, error) {
+	bytes := make([]byte, 16)
+	if _, err := rand.Read(bytes); err != nil {
+		return "", fmt.Errorf("failed to generate random bytes: %w", err)
+	}
+	return hex.EncodeToString(bytes), nil
+}