Add openai codex support

internal/auth/codex/pkce.go

@@ -0,0 +1,47 @@
+package codex
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"fmt"
+)
+
+// GeneratePKCECodes generates a PKCE code verifier and challenge pair
+// following RFC 7636 specifications for OAuth 2.0 PKCE extension
+func GeneratePKCECodes() (*PKCECodes, error) {
+	// Generate code verifier: 43-128 characters, URL-safe
+	codeVerifier, err := generateCodeVerifier()
+	if err != nil {
+		return nil, fmt.Errorf("failed to generate code verifier: %w", err)
+	}
+
+	// Generate code challenge using S256 method
+	codeChallenge := generateCodeChallenge(codeVerifier)
+
+	return &PKCECodes{
+		CodeVerifier:  codeVerifier,
+		CodeChallenge: codeChallenge,
+	}, nil
+}
+
+// generateCodeVerifier creates a cryptographically random string
+// of 128 characters using URL-safe base64 encoding
+func generateCodeVerifier() (string, error) {
+	// Generate 96 random bytes (will result in 128 base64 characters)
+	bytes := make([]byte, 96)
+	_, err := rand.Read(bytes)
+	if err != nil {
+		return "", fmt.Errorf("failed to generate random bytes: %w", err)
+	}
+
+	// Encode to URL-safe base64 without padding
+	return base64.URLEncoding.WithPadding(base64.NoPadding).EncodeToString(bytes), nil
+}
+
+// generateCodeChallenge creates a SHA256 hash of the code verifier
+// and encodes it using URL-safe base64 encoding without padding
+func generateCodeChallenge(codeVerifier string) string {
+	hash := sha256.Sum256([]byte(codeVerifier))
+	return base64.URLEncoding.WithPadding(base64.NoPadding).EncodeToString(hash[:])
+}