[19anveshh]

gh-160732: protect venv prompts from expansion

Fixes #160732.

The generated Bash activation script now keeps the prompt in
VIRTUAL_ENV_PROMPT and expands that variable when the prompt is displayed,
so prompt values containing command substitutions remain literal. The zsh path
uses a psvar entry for the same reason. Deactivation restores the prior zsh
prompt variable.

The regression test creates a venv with a $(touch ...) prompt, starts an
interactive Bash shell, and verifies that the marker is not created and the
prompt remains literal.

Checks:

• python -m py_compile Lib/test/test_venv.py Lib/venv/__init__.py
• bash -n on the generated activation script
• interactive Bash smoke test of the generated activation script
• git diff --check

The full CPython test suite was not run because this environment has no local
CPython build toolchain; the available interpreter is Python 3.13 while this
checkout uses newer CPython-main syntax.

AI tools assisted with issue discovery, code exploration, and drafting. I
reviewed the resulting patch, tests, and PR description manually.

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