Fixing compile issue on Mac (#6538)

Problem
The error of implicit conversion loses integer precision appears in compile time

Summary of Changes
- Updated the CHIPPersistentStorageDelegateBridge.mm file

Test
- Used the gn_build.sh to verify the building is successful

src/darwin/Framework/CHIP/CHIPPersistentStorageDelegateBridge.mm

@@ -105,7 +105,7 @@
  } else {
  error = CHIP_ERROR_NO_MEMORY;
  }
- size = decoded.length();
+ size = static_cast<uint16_t>(decoded.length());
  }
  } else {
  error = CHIP_ERROR_KEY_NOT_FOUND;