Drop RCT_EXPORT_METHOD from RCTAlertManager TurboModule (#57675)

Summary:

Changelog: [Internal]

`RCTAlertManager` is a TurboModule: it conforms to `NativeAlertManagerSpec` and implements `getTurboModule:` returning the codegen'd `...SpecJSI`. For TurboModules, the JS->ObjC dispatch is driven by codegen (the generated spec supplies the `selector` and argument kinds, invoked at runtime via `NSMethodSignature` / `NSInvocation`), not by the `RCT_EXPORT_METHOD` macro's `__rct_export__` metadata.

The exported methods here are async-void with concrete parameter types (no generic `id` requiring `RCTConvert` coercion), so the macro is not functionally required. Convert them to plain ObjC method declarations; conformance to the codegen'd `NativeAlertManagerSpec` protocol keeps compiler-enforced signature parity.

Signature-only refactor with no change to the JS-facing API. Sync methods, methods with `id` params, and `constantsToExport` are intentionally left untouched.

Differential Revision: D113579876

Author: christophpurrer

packages/react-native/React/CoreModules/RCTAlertManager.mm

@@ -65,7 +65,7 @@ - (void)invalidate
  * The key from the `buttons` dictionary is passed back in the callback on click.
  * Buttons are displayed in the order they are specified.
  */
-RCT_EXPORT_METHOD(alertWithArgs : (JS::NativeAlertManager::Args &)args callback : (RCTResponseSenderBlock)callback)
+- (void)alertWithArgs:(JS::NativeAlertManager::Args &)args callback:(RCTResponseSenderBlock)callback
 {
   NSString *title = [RCTConvert NSString:args.title()];
   NSString *message = [RCTConvert NSString:args.message()];