Add explicit type casts to fix compilation errors on Windows. (#31363)

Summary:
Recent changes to `MapBuffer` have broken the compilation on Windows.
This fix is similar to this recently-merged change: #31106

Side note - this PR only addresses a build break, but doesn't address the unsafe casting semantics in `MapBuffer`, which can still cause overflows.

## Changelog

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[General] [Fixed] - Fix compilation errors on Windows.

Pull Request resolved: #31363

Test Plan: RN now builds in Visual Studio on Windows.

Reviewed By: mdvacca

Differential Revision: D28028342

Pulled By: rozele

fbshipit-source-id: 77d8d4870c59b77acfc0ab2f4c3b7df40b59851d

ReactCommon/react/renderer/mapbuffer/MapBufferBuilder.cpp

@@ -127,7 +127,7 @@ void MapBufferBuilder::ensureDynamicDataSpace(int32_t size) {
 }
 
 void MapBufferBuilder::putString(Key key, std::string value) {
-  int32_t strLength = value.length();
+  int32_t strLength = static_cast<int32_t>(value.length());
   const char *cstring = getCstring(&value);
 
   // format [lenght of string (int)] + [Array of Characters in the string]