Support refreshing single files in diagnostic pull

In this code here, workspace diagnostics is repolling every 2 seconds:
https://github.com/microsoft/vscode-languageserver-node/blob/d90a87f9557a0df9142cfb33e251cfa6fe27d970/client/src/common/diagnostic.ts#L440

This makes Pylance continuously do analysis. At least for items that don't seem to save the fact that there are no errors. (Previous result ids don't exist for files that aren't open, but workspace mode asks for all files).

I'm not sure if the polling is as design (might be) but missing the previous result id seems like a bug?

For example, in the issue where we found this:
https://github.com/microsoft/pylance-release/issues/7499

The user has a single file open.

Our result contains all files though. The next poll request only has a previous result id for the file that's open. If all previous results were passed, we would mark the next request as unchanged, but we can't because of the missing result ids for the unopened files.

Start at client/src/common/diagnostic.ts around line 440 and trace the workspace diagnostic pull request and result-ID handling. Use the linked Pylance issue as context for the single-open-file case; done should include a defined way to refresh single files without repeatedly reanalyzing unchanged unopened files.