Handle 50MB limit for multi-message preview

When clicking "Open consumed messages as JSON":

We may silently hit a 50MB file/uri limit set by VS Code, only visible in the Extension Host output channel (when no documents show up after clicking the button above):
```
CodeExpectedError: cannot open confluent.topic.message:-.json?%257B%2522partition%2522%253A-1%252C%2522offset%2522%253A-1%257D. Detail: Files above 50MB cannot be synchronized with extensions.
at T6e.$tryOpenDocument (vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:713:8555)
```
https://github.com/microsoft/vscode/blob/c99a5229bd75503ca45097feb42791a4379a344e/src/vs/workbench/api/browser/mainThreadDocuments.ts#L252

Caused from the migration to a document provider in https://github.com/confluentinc/vscode/pull/1026