Fix stream_context_set_option() mutating the default context

[nicolas-grekas]

Since #20524, _php_stream_open_wrapper_ex() attaches the implicitly substituted default context to context-less streams. A later stream_context_set_option() on such a stream then mutates the global default context, leaking options into every other context-less stream (regression from 8.5):

$a = fopen('php://memory', 'r+');
stream_context_set_option($a, 'http', 'filename', 'test.txt');
var_dump(stream_context_get_options(fopen('php://memory', 'r+')));
// 8.5: [], 8.6: ['http' => ['filename' => 'test.txt']]

Fixed by attaching only explicitly provided contexts. Stream errors are unaffected: they already fall back to the default context when the stream has none.