fix(0.81): use yarn pack + yarn add to match normal install flow

Replace npm install --install-links with yarn pack + yarn add.

npm install <local-path> creates a symlink, and --install-links
copies files but without pre-generated codegen headers. Both
diverge from a normal user's install in ways that break the build:

1. Symlinks cause Node module instance duplication (bundle.js loads
   @react-native/metro-config from the monorepo while metro.config.js
   loads it from the test project — setFrameworkDefaults misses).

2. Missing React/FBReactNativeSpec/ headers (generated by prepack.js,
   shipped in published tarballs). Without them the codegen regenerates,
   but the upstream react-native's FBReactNativeSpec processes first,
   creating the directory — then the fork's version is skipped by
   shouldSkipGenerationForFBReactNativeSpec(), leaving headers without
   macOS-specific fields (critical, modal, defaultInputs).

yarn pack runs the prepack script (generating the codegen headers)
and produces a tarball identical to what npm publish would create.
yarn add installs it as a regular package — no symlinks, correct
headers, matching the normal user flow exactly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Saadnajmi

.github/workflows/microsoft-test-react-native-macos-init.yml

@@ -56,14 +56,15 @@ jobs:
         working-directory: ${{ runner.temp }}/testcli
         run: |
           set -eox pipefail
-          # --install-links copies the package instead of symlinking it.
-          # Without this, Node follows the symlink's real path back to the
-          # monorepo for module resolution, causing bundle.js to load a
-          # different instance of @react-native/metro-config than the one
-          # the user's metro.config.js uses. The setFrameworkDefaults()
-          # call (which registers the "macos" platform) then targets the
-          # wrong instance, and xcodebuild fails with "Invalid platform".
-          npm install --install-links ${{ github.workspace }}/packages/react-native
+          # Pack the local package into a tarball first. This runs the
+          # "prepack" script which generates the FBReactNativeSpec codegen
+          # headers (React/FBReactNativeSpec/) that are shipped in published
+          # releases. Then install from the tarball so the test project
+          # matches a normal user's node_modules layout: no symlinks (which
+          # cause module instance duplication) and pre-generated codegen
+          # headers present (which the build relies on to skip regeneration).
+          yarn --cwd ${{ github.workspace }}/packages/react-native pack -o ${{ runner.temp }}/react-native-macos.tgz
+          yarn add ${{ runner.temp }}/react-native-macos.tgz
 
       - name: Apply macOS template
         working-directory: ${{ runner.temp }}/testcli