Change TypeMapLazyDictionary to use direct cast instead of 'as' operator for RuntimeAssembly (#122885)

Replaced `as RuntimeAssembly` with direct cast `(RuntimeAssembly?)` in
`TypeMapLazyDictionary.CreateMaps()` for both `Assembly.Load()` and
`Assembly.GetEntryAssembly()` calls.

Direct cast surfaces type mismatches as `InvalidCastException` rather
than silently returning null, which is the correct behavior since these
methods should always return `RuntimeAssembly` when non-null in the
runtime environment.

Author: Copilot

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/TypeMapLazyDictionary.cs

@@ -192,11 +192,11 @@ private static unsafe CallbackContext CreateMaps(
             RuntimeAssembly? startingAssembly;
             if (AppContext.GetData("System.Runtime.InteropServices.TypeMappingEntryAssembly") is string entryAssemblyName)
             {
-                startingAssembly = Assembly.Load(entryAssemblyName) as RuntimeAssembly;
+                startingAssembly = (RuntimeAssembly?)Assembly.Load(entryAssemblyName);
             }
             else
             {
-                startingAssembly = Assembly.GetEntryAssembly() as RuntimeAssembly;
+                startingAssembly = (RuntimeAssembly?)Assembly.GetEntryAssembly();
             }
 
             if (startingAssembly is null)