Fix _variables initialization in DocumentSourceLookUp and DocumentSourceUnionWith

This PR addresses issues with variable initialization in the aggregation pipeline stages:

1. In `DocumentSourceLookUp`, moves the initialization of `_variables` and `_variablesParseState` after the creation of `_fromExpCtx` to ensure proper context.

2. In `DocumentSourceUnionWith`:

• Refactors the `UnionWithSharedState` constructor to encapsulate variable initialization
• Removes redundant variable initialization parameters from constructor calls which can lead to dangling IdGenerator pointers.