Additional CI check - preview Bicep diagnostic diffs across quickstarts

It would be nice to ensure that a given type change won't regress diagnostics - to be able to take a snapshot of diagnostics using the quickstarts repo comparing the 'existing' and 'new' behavior on any PR which changes the generated types.

Start by reviewing the repository's existing CI checks and how the quickstarts repository is used for generated types. Define how a PR would capture and compare diagnostics for existing and new behavior, then confirm that a type-changing PR produces a useful diagnostic snapshot and detects regressions.

Written by the indexing model from the issue text.