fix(codegen): use REACT_NATIVE constant for npmPackageName (#2956)

## Summary
Replaces the hardcoded `'react-native-macos'` string at
`packages/react-native/react-native.config.js:188` with the
`REACT_NATIVE` constant from
`scripts/codegen/generate-artifacts-executor/constants.js`. The constant
already reads
`packageJson.name` so the resolved value is unchanged on this fork, but
the indirection means future package-name moves (workspace renames,
internal rebrand, etc.) don't require remembering this one site.

Closes the **"Codegen package name constant"** item on the Road to 0.83
tracking issue (#2901).

## Test Plan

- `pod install` in `packages/rn-tester` runs the codegen step end-to-end
and resolves `npmPackageName` without errors (validated locally on
0.83-merge).
- No behavior change for the current package name `react-native-macos`.