[ty] Make sure to always respond to client requests (#19277)

## Summary

This PR fixes a bug that didn't return a response to the client if the
document snapshotting failed.

This is resolved by making sure that the server always creates the
document snapshot and embed the any failures inside the snapshot.

Closes: astral-sh/ty#798

## Test Plan

Using the test case as described in the linked issue:



https://github.com/user-attachments/assets/f32833f8-03e5-4641-8c7f-2a536fe2e270

crates/ty_server/src/lib.rs

@@ -1,7 +1,7 @@
 use crate::server::{ConnectionInitializer, Server};
 use anyhow::Context;
 pub use document::{NotebookDocument, PositionEncoding, TextDocument};
-pub use session::{DocumentQuery, DocumentSnapshot, Session};
+pub(crate) use session::{DocumentQuery, Session};
 use std::num::NonZeroUsize;
 
 mod document;