Use default settings if initialization options is empty or not provided (#11566)

## Summary

This PR fixes the bug to avoid flattening the global-only settings for
the new server.

This was added in https://github.com/astral-sh/ruff/pull/11497, possibly
to correctly de-serialize an empty value (`{}`). But, this lead to a bug
where the configuration under the `settings` key was not being read for
global-only variant.

By using #[serde(default)], we ensure that the settings field in the
`GlobalOnly` variant is optional and that an empty JSON object `{}` is
correctly deserialized into `GlobalOnly` with a default `ClientSettings`
instance.

fixes: #11507 

## Test Plan

Update the snapshot and existing test case. Also, verify the following
settings in Neovim:

1. Nothing

```lua
ruff = {
  cmd = {
    '/Users/dhruv/work/astral/ruff/target/debug/ruff',
    'server',
    '--preview',
  },
}
```

2. Empty dictionary

```lua
ruff = {
  cmd = {
    '/Users/dhruv/work/astral/ruff/target/debug/ruff',
    'server',
    '--preview',
  },
  init_options = vim.empty_dict(),
}
```

3. Empty `settings`

```lua
ruff = {
  cmd = {
    '/Users/dhruv/work/astral/ruff/target/debug/ruff',
    'server',
    '--preview',
  },
  init_options = {
    settings = vim.empty_dict(),
  },
}
```

4. With some configuration:

```lua
ruff = {
  cmd = {
    '/Users/dhruv/work/astral/ruff/target/debug/ruff',
    'server',
    '--preview',
  },
  init_options = {
    settings = {
      configuration = '/tmp/ruff-repro/pyproject.toml',
    },
  },
}
```

crates/ruff_server/resources/test/fixtures/settings/global_only.json

@@ -1,4 +1,5 @@
 {
+    "settings": {
         "codeAction": {
             "disableRuleComment": {
                 "enable": false
@@ -13,3 +14,4 @@
         "lineLength": 80,
         "exclude": ["third_party"]
     }
+}

crates/ruff_server/src/session/settings.rs

@@ -130,7 +130,7 @@ enum InitializationOptions {
         workspace_settings: Vec<WorkspaceSettings>,
     },
     GlobalOnly {
-        #[serde(flatten)]
+        #[serde(default)]
         settings: ClientSettings,
     },
 }