perf: Faster WritableNativeMap.putString value conversion (#58644)

Summary:
I found (while working on #58611) that `WritableNativeMap::putString` converted its value with fbjni toString(), which calls Java Object.toString() through JNI before running toStdString(). Since the value is already a `jstring`, I call `toStdString()` directly, which produces identical bytes with one fewer JNI call and local reference.

| Value | Before | After | Delta |
|---|---|---|---|
| 11 ASCII | 0.205 | 0.191 | -6.8% |
| 100 ASCII | 0.265 | 0.231 | -12.8% |
| 300 ASCII | 0.451 | 0.418 | -7.3% |
| 1000 ASCII | 0.984 | 0.966 | -1.8% |
| 100 CJK | 0.325 | 0.298 | -8.3% |

## Changelog:
[ANDROID] [CHANGED] - Skip a redundant JNI Object.toString() call in WritableNativeMap.putString.

Pull Request resolved: #58644

Test Plan: I measured putString at 0.614 → 0.474 µs per call for short strings (-23%) on a Galaxy M14, identical output on 21 edge-case strings.

Reviewed By: cortinico

Differential Revision: D121774499

Pulled By: javache

fbshipit-source-id: a20a3d64c6afaf75515f3c3e8173c8e86a1122ce

Author: riteshshukla04

packages/react-native/ReactAndroid/src/main/jni/react/jni/WritableNativeMap.cpp

@@ -55,7 +55,7 @@ void WritableNativeMap::putString(std::string key, alias_ref<jstring> val) {
     return;
   }
   throwIfConsumed();
-  map_.insert(std::move(key), val->toString());
+  map_.insert(std::move(key), val->toStdString());
 }
 
 void WritableNativeMap::putNativeArray(