Stops the `conda activate base` fallback in
`PythonToolchainProvider::activation_script` for two cases:

1. A Conda toolchain whose `environment.name` is `None`.
2. A Conda toolchain whose name is `Some(...)` but `shell.try_quote`
rejects it.

Previously both cases pushed `conda activate base` into the activation
script. With miniforge installed and no env explicitly selected, every
terminal silently activated `(base)`. The user didn't pick `base`;
injecting it is contamination.

Both branches now log a warning and emit no activation line. Users who
want `(base)` can select it as the toolchain explicitly.

Two unit tests cover the new behavior:
`test_conda_activation_skips_when_name_missing` (name=None) and
`test_conda_activation_skips_unquotable_name` (name=Some but unquotable,
exercised with an embedded NUL byte that `shlex::try_quote` rejects).
The existing `test_conda_activation_script_injection` still verifies
that quotable names get activated.

Release Notes:

- Fixed Zed silently injecting `conda activate base` into terminals when
a Conda manager (miniforge/miniconda) was installed but no specific
environment was selected.