fix: catch notebook snapshot errors in chat editing (#301202)

When NotebookTextModel.createSnapshot() throws 'Notebook too large to
backup' (outputs exceed notebook.backup.sizeLimit), catch the error at
the caller side instead of letting it propagate unhandled.

- getCurrentSnapshot(): Catch error, log via loggingService, fall back
  to initialContent.
- createSnapshot(): Reuse getCurrentSnapshot() for the modified model
  snapshot. Catch error for the original model snapshot and fall back
  to initialContent.
- create(): Let error propagate to the session's existing try/catch.
- Revert model-level changes (NotebookTextModel still throws).
- Log only error messages, not URIs, to avoid privacy concerns.