docs: use actual lowercase parameter names in error messages

[vprusso]

Some user-facing error messages refer to parameters by uppercase MATLAB/QETLAB-style names (PHI, DIM) instead of the actual lowercase Python parameter names (phi, dim). This is confusing: a user who hits the error cannot map PHI to any argument they passed.

What to do

Update the messages to use the real parameter names.

• toqito/channel_props/channel_dim.py — four messages using PHI/DIM, e.g. "The input and output spaces of PHI must be square." becomes "The input and output spaces of `phi` must be square."
• toqito/perms/permute_systems.py — the message mentioning DIM and X.
• toqito/states/horodecki.py — "InvalidDim: DIM must be one of [3, 3], or [2, 4]." becomes dim.

Find them with:

git grep -nE "PHI|DIM" toqito/channel_props/channel_dim.py toqito/perms/permute_systems.py toqito/states/horodecki.py

Note

Some tests assert on these exact strings via pytest.raises(..., match=...); update those matches too. Then run:

uv run pytest toqito/channel_props/tests/test_channel_dim.py toqito/perms/tests toqito/states/tests/test_horodecki.py