Add an abstract base class type for session argument in context manager

### 📝 Description of the feature

The "session" argument could have an abstract base class type that would show up in intellisense.

using(....

### 💡 Steps for implementing the feature

Create an Abstract base class for session type (for solver, meshing or FileSession)

### 🔗 Useful links and references

_No response_

Start by locating the session argument types used by solver, meshing, and FileSession context managers; the issue does not name specific files or tests. Define the shared abstract session type so it is exposed for IntelliSense, and verify that all three context-manager variants use it consistently.

Written by the indexing model from the issue text.