refactor(xtab): extract _performFetch from _streamEditsImpl
Separate the HTTP fetch lifecycle (FetchStreamSource setup, makeChatRequest2,
initial error handling, line stream construction with cursor-tag removal and
latency logging) into a dedicated _performFetch method.
This makes the fetch infrastructure self-contained and returns a clean
AsyncIterable<string> line stream + getFetchFailure callback for downstream
format handlers, matching the pattern already used by
XtabCustomDiffPatchResponseHandler.
_streamEditsImpl is now a coordinator: it calls _performFetch, then dispatches
to format handlers, then post-processes edit-window results.
Part of #308744
Ulugbek Abdullaev
c5ac25e125