gh-140006: Preserve fish prompt status in venv activation

[ThanhNguyxn]

Summary

• Replace echo "exit $old_status" | . with return $old_status in activate.fish so the fish prompt preserves the previous command status without sourcing exit from stdin.
• Add test_activate_fish_script_preserves_status in Lib/test/test_venv.py to assert the generated script contains the fixed prompt-status line.
• Add a Library NEWS entry for gh-140006.

Verification

• python -m py_compile Lib/test/test_venv.py
• python -c "import importlib.util, pathlib, tempfile, os; spec=importlib.util.spec_from_file_location('repo_venv', r'D:\Code\cpython\Lib\venv\__init__.py'); mod=importlib.util.module_from_spec(spec); spec.loader.exec_module(mod); d=tempfile.mkdtemp(); mod.create(d, with_pip=False, scm_ignore_files=frozenset()); p=pathlib.Path(d)/('Scripts' if os.name=='nt' else 'bin')/'activate.fish'; s=p.read_text(encoding='utf-8'); assert 'return $old_status' in s; assert 'echo \"exit $old_status\" | .' not in s; print('ok')"

[python-cla-bot]

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