fix(api): don't try to get/set option for invalid option name (#31302)

Problem:
`validate_option_value_args()` returns `OK` even if option name is
invalid or if option doesn't have the supported scope, which leads to
Neovim still trying to erroneously get/set the option in those cases,
which can lead to an assertion failure when `option_has_scope()` is
invoked. This issue miraculously doesn't exist in release builds since
the assertion is skipped and `(get/set)_option_value_for` returns if
there is an error set, but that is not the intended location for that
error to be caught.

Solution:
Make `validate_option_value_args()` return `FAIL` if there is an error
set, which causes the API option functions to return early instead of
trying to get/set an invalid option.

src/nvim/api/options.c

@@ -95,7 +95,7 @@ static int validate_option_value_args(Dict(option) *opts, char *name, OptIndex *
     }
   }
 
-  return OK;
+  return ERROR_SET(err) ? FAIL : OK;
 #undef HAS_KEY_X
 }