Reset ruff.configuration if it contains VS Code variables (#746)

## Summary

fixes: #741 

## Test Plan

`settings.json`:

```json
{
  "ruff.configuration": "${workspaceFolder}/formatter/ruff.toml"
}
```

"Ruff" output channel:

> [!NOTE]
>
> I've only kept the necessary settings here but the actual log output
includes everything

```
2025-05-01 10:38:43.029 [info] Workspace settings for /Users/dhruv/playground/ruff: {
    "nativeServer": "on",
    "cwd": "/Users/dhruv/playground/ruff",
    "workspace": "file:///Users/dhruv/playground/ruff",
    "path": [
        "/Users/dhruv/work/astral/ruff/target/debug/ruff"
    ],
    "configuration": "/Users/dhruv/playground/ruff/formatter/ruff.toml",
}
2025-05-01 10:38:43.030 [info] Resetting 'ruff.configuration' to null in global settings because it contains workspace specific variables
2025-05-01 10:38:43.030 [info] Global settings: {
    "nativeServer": "on",
    "cwd": "/",
    "workspace": "/",
    "path": [
        "/Users/dhruv/work/astral/ruff/target/debug/ruff"
    ],
    "configuration": null,
}
```

`settings.json`:

```json
{
	"ruff.configuration": "${workspaceFolder:root}/formatter/ruff.toml"
}
```

"Ruff" output channel:

```
2025-05-01 10:40:42.200 [info] Workspace settings for /Users/dhruv/playground/ruff: {
    "nativeServer": "on",
    "cwd": "/Users/dhruv/playground/ruff",
    "workspace": "file:///Users/dhruv/playground/ruff",
    "path": [
        "/Users/dhruv/work/astral/ruff/target/debug/ruff"
    ],
    "configuration": "${workspaceFolder:root}/formatter/ruff.toml",
}
2025-05-01 10:40:42.200 [info] Resetting 'ruff.configuration' to null in global settings because it contains workspace specific variables
2025-05-01 10:40:42.200 [info] Global settings: {
    "nativeServer": "on",
    "cwd": "/",
    "workspace": "/",
    "path": [
        "/Users/dhruv/work/astral/ruff/target/debug/ruff"
    ],
    "configuration": null,
}
```

For this specific test case, you can see that the workspace settings
also includes the `${workspaceFolder:root}` because it doesn't exists.
That would also create an error:

```
2025-05-01 10:42:14.245925000 ERROR Failed to load settings from `configuration`: error looking key 'workspaceFolder:root' up: environment variable not found
```

I think we should handle that as well.

Author: dhruvmanila

src/common/settings.ts

@@ -196,16 +196,36 @@ function getOptionalGlobalValue<T>(config: WorkspaceConfiguration, key: string):
   return inspect?.globalValue;
 }
 
+/**
+ * Returns the global settings for the extension.
+ *
+ * ## Notes
+ *
+ * The global settings do not belong to a specific workspace. This means that
+ * variables such as `${workspaceFolder}` or `${workspaceFolder:...}` are not
+ * resolved. The language server does not support these variables, so they are
+ * filtered out. For example, if `configuration` has either of these variables,
+ * it will be set to `null`.
+ */
 export async function getGlobalSettings(namespace: string): Promise<ISettings> {
   const config = getConfiguration(namespace);
+
+  let configuration = getGlobalValue<string | object | null>(config, "configuration", null);
+  if (typeof configuration === "string" && configuration.search(/\$\{workspaceFolder/) !== -1) {
+    logger.info(
+      `Resetting '${namespace}.configuration' to null in global settings because it contains workspace specific variables`,
+    );
+    configuration = null;
+  }
+
   return {
     nativeServer: getGlobalValue<NativeServer>(config, "nativeServer", "auto"),
     cwd: process.cwd(),
     workspace: process.cwd(),
     path: getGlobalValue<string[]>(config, "path", []),
     ignoreStandardLibrary: getGlobalValue<boolean>(config, "ignoreStandardLibrary", true),
     interpreter: [],
-    configuration: getGlobalValue<string | object | null>(config, "configuration", null),
+    configuration,
     importStrategy: getGlobalValue<ImportStrategy>(config, "importStrategy", "fromEnvironment"),
     codeAction: getGlobalValue<CodeAction>(config, "codeAction", {}),
     lint: {