chat-lib: preserve target document URI on NES results (#319317)

[chat-lib] Preserve target document URI on NES results

The NES facade exposed in chat-lib was dropping the target document id
when constructing the public NESResult, so cross-file Next Edit
Suggestions could not be routed to the correct document downstream
(notably the Copilot Language Server / Visual Studio).

This forwards `targetDocumentId.uri` from the internal next-edit result
as an optional `targetDocumentUri` on `INESResult.result`. When the
field is omitted, consumers continue to apply the edit to the requested
document, preserving backward compatibility for single-file callers.

Refs https://github.com/microsoft/vscode-internalbacklog/issues/7835

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>