ruff server: Fix multiple issues with Neovim and Helix (#11497)

## Summary

Fixes https://github.com/astral-sh/ruff/issues/11236.

This PR fixes several issues, most of which relate to non-VS Code
editors (Helix and Neovim).

1. Global-only initialization options are now correctly deserialized
from Neovim and Helix
2. Empty diagnostics are now published correctly for Neovim and Helix.
3. A workspace folder is created at the current working directory if the
initialization parameters send an empty list of workspace folders.
4. The server now gracefully handles opening files outside of any known
workspace, and will use global fallback settings taken from client
editor settings and a user settings TOML, if it exists.

## Test Plan

I've tested to confirm that each issue has been fixed.

* Global-only initialization options are now correctly deserialized from
Neovim and Helix + the server gracefully handles opening files outside
of any known workspace


4f33477f-20c8-4e50-8214-6608b1a1ea6b

* Empty diagnostics are now published correctly for Neovim and Helix


c93f56a0-f75d-466f-9f40-d77f99cf0637

* A workspace folder is created at the current working directory if the
initialization parameters send an empty list of workspace folders.



b4b2e818-4b0d-40ce-961d-5831478cc726

crates/ruff_server/src/session.rs

@@ -67,7 +67,7 @@ impl Session {
         Some(DocumentSnapshot {
             resolved_client_capabilities: self.resolved_client_capabilities.clone(),
             client_settings: self.index.client_settings(&key, &self.global_settings),
-            document_ref: self.index.make_document_ref(key)?,
+            document_ref: self.index.make_document_ref(key, &self.global_settings)?,
             position_encoding: self.position_encoding,
         })
     }