Preserve explicit Paramiko username

[Herrtian]

Fixes #643.

When Paramiko loads ssh config defaults, a Host * user should not replace a username that was already supplied in the host spec, like paramiko://vagrant@127.0.0.1:2222.

This keeps the ssh config user fallback for host specs without a user, and adds a small regression test for both paths.

Checked with:

uv run --with paramiko python -
uv run --with ruff ruff check testinfra/backend/paramiko.py test/test_backends.py
uv run --with mypy --with types-paramiko --with paramiko --with pytest mypy testinfra/backend/paramiko.py

I also tried the focused pytest test on Windows, but this repo's test conftest hits Docker probing before collection and currently trips a Windows bytes-args issue in the local backend, so I verified the Paramiko config path directly instead.

[Herrtian]

Pushed 24ebb76 to fix the mypy failure from the now-unused type ignores. I checked tox -e mypy, tox -e lint, and git diff --check locally. The remaining py311 failure is in the existing Ansible encoding test, so I left that out of this Paramiko-focused change.