fix: wrong path on migrated venv - #2996

Preserve the existing Bash activation fallback while safely deriving the
script path in Bash, Zsh, and KornShell. Avoid leaking temporary shell
state and keep regular activation compatible with nounset and Yash.

This fixes activation state when the recorded environment path is
missing. It does not restore --relocatable, rewrite installed scripts,
or promise that an entire environment can be moved.

Upstream context: pypa#2996, pypa#3090,
pypa#3091.
Downstream issues: astral-sh/uv#7480, astral-sh/uv#12662.
Downstream fixes: astral-sh/uv#5640, astral-sh/uv#12672,
astral-sh/uv#20743.

Retain the recorded environment path while it exists and fall back to
the sourced Fish script only when it does not. Test VIRTUAL_ENV, PATH,
and working-directory preservation for both cases.

This aligns Fish with the existing Bash activation fallback; it does
not reintroduce relocatable virtual environments.

Upstream context: pypa#2996, pypa#3091.
Downstream issue: astral-sh/uv#19631.
Downstream fixes: astral-sh/uv#5515, astral-sh/uv#19856.

Preserve the recorded environment directory when it still exists.
Capture the activation module path at parse time and use it only as a
fallback. Test overlay activation with existing and missing paths.

This aligns Nushell with the existing Bash activation fallback; it does
not reintroduce relocatable virtual environments.

Upstream context: pypa#2996, pypa#3091.
Downstream issue: astral-sh/uv#16973.
Downstream fixes: astral-sh/uv#17036, astral-sh/uv#20743.

Keep the recorded environment directory while it exists and derive its
absolute path from activate.bat only as a fallback. Test both existing
and missing paths without changing batch-file encoding.

This aligns Windows batch with the existing Bash activation fallback;
it does not reintroduce relocatable virtual environments.

Upstream context: pypa#2996, pypa#3091.
Downstream issue: astral-sh/uv#3863.
Downstream fix: astral-sh/uv#5515.