Skip RawPropsParser::prepare<T>() when the iterator-setter path is active (#57329)

Summary:
Pull Request resolved: #57329

`RawPropsParser::prepare<ConcreteProps>()` runs in `ConcreteComponentDescriptor`'s constructor and builds the parser's `keys_` vector + `nameToIndex_` length-bucketed map by walking every `convertRawProp` call in a probe construction of `ConcreteProps`. The cost is O(n²) in the number of props (the comment in `RawPropsParser::at` notes 4950 lookups for a 100-prop class) and is paid once per component class at app startup.

Those data structures are consumed **only** by `RawProps::at()`, which is reached exclusively through the classic per-field `convertRawProp` path. The iterator-setter path skips `parse()` entirely (see the prior diff in the stack), so the prepared parser is dead weight when both:

1. `HasIteratorSetterCtor<ConcreteProps>` is satisfied (i.e. the type opts into the iterator-setter path), AND
2. `enableCppPropsIteratorSetter()` is on at runtime.

Guard the call accordingly. Classes that don't satisfy the concept always run `prepare<T>()` (they can only use the classic path); when the runtime flag is off, all classes run it (since they all fall back to classic).

Changelog:
[Internal]

Differential Revision: D109569571

Author: javache

packages/react-native/ReactCommon/react/renderer/core/ConcreteComponentDescriptor.h

@@ -48,7 +48,18 @@ class ConcreteComponentDescriptor : public ComponentDescriptor {
       RawPropsParser &&rawPropsParser = {})
       : ComponentDescriptor(parameters, std::move(rawPropsParser))
   {
-    rawPropsParser_.prepare<ConcreteProps>();
+    // The parser's `keys_` / `nameToIndex_` are only consumed by
+    // `RawProps::at()`, which is reached exclusively through the classic
+    // per-field `convertRawProp` path. When `ConcreteProps` opts into the
+    // iterator-setter path and the runtime flag is on, `parse()` is never
+    // called, so the O(n²) preparation here is wasted. Skip it.
+    if constexpr (HasIteratorSetterCtor<ConcreteProps>) {
+      if (!ReactNativeFeatureFlags::enableCppPropsIteratorSetter()) {
+        rawPropsParser_.prepare<ConcreteProps>();
+      }
+    } else {
+      rawPropsParser_.prepare<ConcreteProps>();
+    }
   }
 
   ComponentHandle getComponentHandle() const override