nes: patch: fix: don't yield malformed edits on fetch fail/cancellation

[ulugbekna]

No description provided.

[Copilot]

Pull request overview

This PR updates the Xtab “custom diff patch” streaming path to stop emitting edits once the underlying chat fetch has failed or been cancelled, preventing consumers from applying malformed/partial edits.

Changes:

• Extend XtabCustomDiffPatchResponseHandler.handleResponse with an optional getFetchFailure callback and early-return when it reports a failure/cancellation.
• Wire the handler up from XtabProvider by passing a callback derived from chatResponseFailure.
• Add unit tests for cancellation/failure behavior in the custom diff patch response handler.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/extension/xtab/node/xtabCustomDiffPatchResponseHandler.ts	Adds fetch-failure detection hook to stop yielding edits on cancellation/failure.
src/extension/xtab/node/xtabProvider.ts	Passes fetch-failure mapping callback into the custom diff patch response handler.
src/extension/xtab/test/node/xtabCustomDiffPatchResponseHandler.spec.ts	Adds tests intended to validate stopping edit emission on fetch failure/cancellation.