uv pip uninstall does not support the -y (assume-yes) flag. Remove it
from the uv fallback argv while keeping it for the python -m pip
uninstall path.
Reported-by: chengyongru
CI's uv-managed Python does not have pip importable, so the runtime
falls back to uv pip. Four tests that verify the python -m pip path
were failing because _pip_available() returned False in CI.
Monkeypatch _pip_available to True in tests that intentionally verify
the pip code path, so they pass regardless of the CI Python
environment.
When nanobot is installed via uv tool install, sys.executable points to
a Python that does not have pip available as a module. _pip_install_argv
and _pip_uninstall_argv always used [sys.executable, -m, pip, ...]
which fails in that environment.
Add _pip_available() helper that checks importlib.util.find_spec('pip').
When pip is not available and uv is on PATH, fall back to:
uv pip install --python <sys.executable> ...
uv pip uninstall --python <sys.executable> -y ...
If neither pip nor uv is available, raise CliAppError.
Fixes#4158
