Cache writes faster; relay multipart subscriptions no longer fail on serialization error

Patch Changes

• #13315 a406cc9 Thanks @fallintoplace! - Prevent relay multipart subscriptions from issuing a fetch request after serializing the request body fails.

• #13307 abd0781 Thanks @wolfie! - Speed up cache writes by avoiding a full AST visit of every written field to detect @stream. The check now runs only when the result carries stream info, and only inspects the field node's own directives. As a result, fields that merely contain @stream on a nested field are no longer treated as streamed themselves and now overwrite existing lists like regular fields instead of merging chunk-wise.