gh-159498: Fix PowerShell venv activation for paths containing brackets

[repzakske]

Fixes #159498.

PowerShell interprets square brackets in the activation script's path and config path as wildcard syntax. This can prevent activation under project [2026]/.venv, or make venv[1] select a sibling directory named venv1.

Use literal lookups for the script directory and pyvenv.cfg, constructing the config path without wildcard resolution.

The regression test activates an environment named venv[1] in each available PowerShell implementation and checks the active environment, configured prompt, and selected interpreter using the existing subprocess helper.

Validation

• The regression test fails against the original script and when each corrected lookup is independently reverted, in both shells.
• Built CPython main locally and ran python -m test -v -u cpu test_venv: 54 tests, 9 platform skips, no failures.
• Tested with PowerShell 7.6.5 and Windows PowerShell 5.1 on Windows.
• Applicable pre-commit hooks and patchcheck passed.
• Full CPython test-suite and upstream CI results remain pending.

Please consider backports to 3.14 and 3.15.

• Issue: PowerShell venv activation treats square brackets in paths as wildcards #159498

[python-cla-bot]

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