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

crates/ty_server/src/server/api/diagnostics.rs

@@ -80,7 +80,7 @@ pub(super) fn publish_diagnostics(
     let path = key.path();
 
     let snapshot = session
-        .take_snapshot(url.clone())
+        .take_document_snapshot(url.clone())
         .ok_or_else(|| anyhow::anyhow!("Unable to take snapshot for document with URL {url}"))
         .with_failure_code(lsp_server::ErrorCode::InternalError)?;