[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/server/api/diagnostics.rs

@@ -10,11 +10,10 @@ use ruff_db::files::FileRange;
 use ruff_db::source::{line_index, source_text};
 use ty_project::{Db, ProjectDatabase};
 
-use super::LSPResult;
 use crate::document::{DocumentKey, FileRangeExt, ToRangeExt};
-use crate::server::Result;
+use crate::session::DocumentSnapshot;
 use crate::session::client::Client;
-use crate::{DocumentSnapshot, PositionEncoding, Session};
+use crate::{PositionEncoding, Session};
 
 /// Represents the diagnostics for a text document or a notebook document.
 pub(super) enum Diagnostics {
@@ -64,30 +63,29 @@ pub(super) fn clear_diagnostics(key: &DocumentKey, client: &Client) {
 /// This function is a no-op if the client supports pull diagnostics.
 ///
 /// [publish diagnostics notification]: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_publishDiagnostics
-pub(super) fn publish_diagnostics(
-    session: &Session,
-    key: &DocumentKey,
-    client: &Client,
-) -> Result<()> {
+pub(super) fn publish_diagnostics(session: &Session, key: &DocumentKey, client: &Client) {
     if session.client_capabilities().pull_diagnostics {
-        return Ok(());
+        return;
     }
 
     let Some(url) = key.to_url() else {
-        return Ok(());
+        return;
     };
 
-    let path = key.path();
+    let snapshot = session.take_document_snapshot(url.clone());
 
-    let snapshot = session
-        .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)?;
+    let document = match snapshot.document() {
+        Ok(document) => document,
+        Err(err) => {
+            tracing::debug!("Failed to resolve document for URL `{}`: {}", url, err);
+            return;
+        }
+    };
 
-    let db = session.project_db_or_default(path);
+    let db = session.project_db_or_default(key.path());
 
     let Some(diagnostics) = compute_diagnostics(db, &snapshot) else {
-        return Ok(());
+        return;
     };
 
     // Sends a notification to the client with the diagnostics for the document.
@@ -95,7 +93,7 @@ pub(super) fn publish_diagnostics(
         client.send_notification::<PublishDiagnostics>(PublishDiagnosticsParams {
             uri,
             diagnostics,
-            version: Some(snapshot.query().version()),
+            version: Some(document.version()),
         });
     };
 
@@ -109,25 +107,28 @@ pub(super) fn publish_diagnostics(
             }
         }
     }
-
-    Ok(())
 }
 
 pub(super) fn compute_diagnostics(
     db: &ProjectDatabase,
     snapshot: &DocumentSnapshot,
 ) -> Option<Diagnostics> {
-    let Some(file) = snapshot.file(db) else {
-        tracing::info!(
-            "No file found for snapshot for `{}`",
-            snapshot.query().file_url()
-        );
+    let document = match snapshot.document() {
+        Ok(document) => document,
+        Err(err) => {
+            tracing::info!("Failed to resolve document for snapshot: {}", err);
+            return None;
+        }
+    };
+
+    let Some(file) = document.file(db) else {
+        tracing::info!("No file found for snapshot for `{}`", document.file_path());
         return None;
     };
 
     let diagnostics = db.check_file(file);
 
-    if let Some(notebook) = snapshot.query().as_notebook() {
+    if let Some(notebook) = document.as_notebook() {
         let mut cell_diagnostics: FxHashMap<Url, Vec<Diagnostic>> = FxHashMap::default();
 
         // Populates all relevant URLs with an empty diagnostic list. This ensures that documents