feat(context-engine): plumb sessionKey into all ContextEngine methods (#44157)

Merged via squash.

Prepared head SHA: 0b341f6f4c
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Reviewed-by: @jalehman

src/shared/global-singleton.ts

@@ -1,8 +1,7 @@
 export function resolveGlobalSingleton<T>(key: symbol, create: () => T): T {
   const globalStore = globalThis as Record<PropertyKey, unknown>;
-  const existing = globalStore[key] as T | undefined;
   if (Object.prototype.hasOwnProperty.call(globalStore, key)) {
-    return existing;
+    return globalStore[key] as T;
   }
   const created = create();
   globalStore[key] = created;