fix(doctor): use sys.executable for pip show in _pip_version

[moonandlife]

Summary

Fix hust-ascend-manager setup / doctor failing on minimal Ascend
runtimes where the conda env only exposes python3 (not python).

Root cause

In src/hust_ascend_manager/doctor.py, _pip_version was hardcoding
the interpreter name:

def _pip_version(pkg: str) -> str | None:
    rc, out, _ = _run(["python", "-m", "pip", "show", pkg])

On runners where the active env ships only python3 (typical of
Euleros/Ascend base images and minimal CI runners), this raises:

FileNotFoundError: [Errno 2] No such file or directory: 'python'

The traceback was observed in CI when setup invokes
doctor.collect_report() (see cli.py:140 → setup.py:220 →
doctor.py:703 → _pip_version).

Fix

Replace the hardcoded "python" with sys.executable. This is
guaranteed to:

• be the actual interpreter that imported the manager (so it always
  exists as a real path)
• have pip available in the same env
• be the right interpreter for conda envs, venvs, and containers,
  regardless of whether python, python3, or python3.11 is in
  PATH

No new imports needed (sys is already imported at the top of
doctor.py).

Scope

Single-line change in _pip_version. Sibling module runtime.py
already handles the same concern with an explicit
("python3", "python") fallback list — sys.executable is the
more robust version of that pattern.

Verification

• pytest tests/ -q: 66 passed (no test changes needed — they mock
  _pip_version directly).
• _pip_version("pip") returns the actual version
  ('26.1.2' in the dev env).
• _pip_version("not-a-real-pkg") returns None (unchanged
  behaviour on missing packages).

Related

This is the same class of issue that already exists in
runtime.py:38 (for candidate in ("python3", "python")), but
the doctor path was missed. This PR only fixes the doctor path. The
runtime path can be unified with sys.executable in a follow-up
if desired.

Assisted-by: Qoder Agent