Rename to `SessionSnapshot`, move unwind assertion closer (#19177)
This PR addresses the post-merge review comments from
https://github.com/astral-sh/ruff/pull/19041, specifically it:
- Rename `WorkspaceSnapshot` to `SessionSnapshot`
- Rename `take_workspace_snapshot` to `take_session_snapshot`
- Rename `take_snapshot` to `take_document_snapshot`
- Move `AssertUnwindSafe` closer to the `catch_unwind` call which
requires the assertion
