: config: global: robust to drop order in test overrides (#1457)

Summary:

`override_key` now pushes frames onto a per-key stack with unique tokens, allowing out-of-order drops and deterministic restoration. `ConfigValueGuard::drop` pops the active frame, restores the previous or default value, and updates the mirrored env var only when the top changes. `ensure_test_override_layer_mut` removed as redundant. `Key::name` moved to an unconstrained impl to relax the `Named` bound.

adds tests verifying LIFO restoration and stability under out-of-order drops, covering both attrs and env behavior.

Differential Revision: D84085259

Author: shayne-fletcher

hyperactor/src/attrs.rs

@@ -153,6 +153,13 @@ pub struct Key<T: 'static> {
     attrs: &'static LazyLock<Attrs>,
 }
 
+impl<T> Key<T> {
+    /// Returns the name of this key.
+    pub fn name(&self) -> &'static str {
+        self.name
+    }
+}
+
 impl<T: Named + 'static> Key<T> {
     /// Creates a new key with the given name.
     pub const fn new(
@@ -167,11 +174,6 @@ impl<T: Named + 'static> Key<T> {
         }
     }
 
-    /// Returns the name of this key.
-    pub fn name(&self) -> &'static str {
-        self.name
-    }
-
     /// Returns a reference to the default value for this key, if one exists.
     pub fn default(&self) -> Option<&'static T> {
         self.default_value