Improve BasicAuth middleware: use strings.Cut and RFC compliance

- Replace manual for loop with strings.Cut for credential parsing
- Simplify realm handling to always quote according to RFC 7617
- Improve code readability and maintainability

Fixes #2794

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

middleware/basic_auth.go

@@ -84,27 +84,21 @@ func BasicAuthWithConfig(config BasicAuthConfig) echo.MiddlewareFunc {
 				}
 
 				cred := string(b)
-				for i := 0; i < len(cred); i++ {
-					if cred[i] == ':' {
-						// Verify credentials
-						valid, err := config.Validator(cred[:i], cred[i+1:], c)
-						if err != nil {
-							return err
-						} else if valid {
-							return next(c)
-						}
-						break
+				user, pass, ok := strings.Cut(cred, ":")
+				if ok {
+					// Verify credentials
+					valid, err := config.Validator(user, pass, c)
+					if err != nil {
+						return err
+					} else if valid {
+						return next(c)
 					}
 				}
 			}
 
-			realm := defaultRealm
-			if config.Realm != defaultRealm {
-				realm = strconv.Quote(config.Realm)
-			}
-
 			// Need to return `401` for browsers to pop-up login box.
-			c.Response().Header().Set(echo.HeaderWWWAuthenticate, basic+" realm="+realm)
+			// Realm is case-insensitive, so we can use "basic" directly. See RFC 7617.
+			c.Response().Header().Set(echo.HeaderWWWAuthenticate, basic+" realm="+strconv.Quote(config.Realm))
 			return echo.ErrUnauthorized
 		}
 	}