fix(ci): platform-aware pipx detection test (Windows matrix)

[curdriceaurora]

Summary

Restores green status of the CI Full Matrix / Test Windows (Python 3.12) job, which has been the sole failing matrix slot on recent scheduled runs (e.g. run 26871404355 on commit dd13452).

Failure detected

Single deterministic test failure, only on Windows + Python 3.12:

FAILED tests/cli/test_doctor.py::TestInstallMethodDetection::test_detect_pipx_via_pipx_home_env
    - AssertionError: assert 'pip' == 'pipx'

All four other matrix jobs (Linux py3.11, Linux py3.12, macOS py3.12, coverage gate) pass on the same commit.

Root cause

The test (lines 254–262 of tests/cli/test_doctor.py) hardcoded POSIX-style paths:

custom_home = "/custom/pipx"
fake_exe    = "/custom/pipx/venvs/fo-core/bin/python"

But the production code _detect_install_method() in src/cli/doctor.py:119 builds its comparison prefix with the platform's join + separator:

if exe_path.startswith(os.path.join(pipx_home, "venvs") + os.sep):

On Windows this evaluates to "/custom/pipx\venvs\" (mixed separators because pipx_home is forward-slash but os.sep == "\\"). The startswith check against a pure forward-slash exe_path always fails, so the function returns "pip" and the assertion blows up.

This is a test-side bug — the production code's behaviour on real Windows installs (where both PIPX_HOME and sys.executable use \\) is correct. The other tests in the same class already use either platform-mocked os.path.expanduser or os.path.expanduser(...) directly, which is why they pass on Windows.

Fix

Build the test inputs with os.sep + os.path.join so the same separator semantics are exercised on POSIX and Windows:

custom_home = os.sep + os.path.join("custom", "pipx")
fake_exe    = os.path.join(custom_home, "venvs", "fo-core", "bin", "python")

• POSIX: custom_home="/custom/pipx", prefix "/custom/pipx/venvs/" → matches ✓
• Windows: custom_home="\\custom\\pipx", prefix "\\custom\\pipx\\venvs\\" → matches ✓

Verification

Local cross-platform simulation using posixpath and ntpath modules:

Inputs	POSIX	Windows
New (os.sep-aware)	pipx ✓	pipx ✓
Old (hardcoded /)	pipx ✓	pip ✗ (reproduces failure)

Other local checks:

• ruff check tests/cli/test_doctor.py — passes
• ruff format --check — already formatted
• G1 (pre-commit-staged absolute-path) — clean
• G2 (scripts/check_test_hardcoded_paths.py full-suite scan) — clean

The Linux/macOS jobs of the same Matrix workflow already passed on the unchanged production code, so they will continue to pass under the new test inputs (POSIX path → same startswith match).

Trade-offs

• No production code change — the implementation is correct on real Windows installs (where PIPX_HOME would itself use \\). The fix is scoped to the test simulating realistic per-platform inputs.
• No test weakening — the assertion still verifies that a PIPX_HOME-based pipx executable is detected as "pipx"; only the input construction is now platform-aware.

Impacted areas

• tests/cli/test_doctor.py — single test function (test_detect_pipx_via_pipx_home_env), +6/-2.

Links

• Failing run: https://github.com/curdriceaurora/fo-core/actions/runs/26871404355
• Failing job: https://github.com/curdriceaurora/fo-core/actions/runs/26871404355/job/79247377805

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