Run cross-platform parity test on Windows and macOS in CI (#6241)

.github/workflows/cross-platform-parity-ci.yml

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+# SPDX-License-Identifier: AGPL-3.0-only
+# Copyright 2026-present the Unsloth AI Inc. team. All rights reserved.
+
+# Runs tests/python/test_cross_platform_parity.py on Windows and macOS.
+#
+# Why: that test is the guard that install.sh and install.ps1 stay in
+# sync, but today it only runs on ubuntu-latest (auto-discovered by
+# studio-backend-ci.yml's "Repo tests (CPU)" job). The test reads both
+# installer scripts, and on Windows Path.read_text() defaults to the
+# cp1252 locale encoding, so a non-cp1252 byte in install.sh (it already
+# contains a U+274C) raises UnicodeDecodeError there even though Linux and
+# macOS default to UTF-8. The reads were pinned to encoding="utf-8" in
+# #6166; this job keeps that from silently regressing by exercising the
+# test on the platforms it claims parity for. Pure pytest, no GPU,
+# sub-second, so the matrix is cheap.
+
+name: Cross-platform parity
+
+on:
+  pull_request:
+    paths:
+      - 'install.sh'
+      - 'install.ps1'
+      - 'tests/python/test_cross_platform_parity.py'
+      - '.github/workflows/cross-platform-parity-ci.yml'
+  push:
+    branches: [main]
+    paths:
+      - 'install.sh'
+      - 'install.ps1'
+      - 'tests/python/test_cross_platform_parity.py'
+      - '.github/workflows/cross-platform-parity-ci.yml'
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  parity:
+    name: parity (${{ matrix.os }})
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [windows-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2
+        with:
+          persist-credentials: false
+      - uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405  # v6.2.0
+        with:
+          python-version: '3.12'
+          cache: 'pip'
+      - run: python -m pip install -U pip pytest
+      - name: Cross-platform parity test
+        run: python -m pytest tests/python/test_cross_platform_parity.py -q