Fix: Resolve Matrix CI failure (Windows pipx detection)

[curdriceaurora]

Summary

The scheduled CI Full Matrix workflow has been failing daily on main since at least 2026-06-02 — the Test Windows (Python 3.12) job fails one test while all other matrix variants (Linux 3.11, Linux 3.12, macOS 3.12) pass.

Run	Conclusion	Failing job
#26938361854 (2026-06-04)	failure	Test Windows (py3.12)
#26871404355 (2026-06-03)	failure	Test Windows (py3.12)
#26806177119 (2026-06-02)	failure	Test Windows (py3.12)

Failing test (1 of 4922 total):

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

Root Cause Analysis

The test sets PIPX_HOME=/custom/pipx and sys.executable=/custom/pipx/venvs/fo-core/bin/python, then expects _detect_install_method() to return "pipx".

cli/doctor.py:117-120 builds the match pattern via:

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

On Windows:

• os.path.join("/custom/pipx", "venvs") → "/custom/pipx\venvs" (mixed separators; join doesn't rewrite the input's existing forward slashes but uses os.sep to attach the new segment)
• os.sep → "\\"
• Pattern → "/custom/pipx\venvs\"
• "/custom/pipx/venvs/fo-core/bin/python".startswith("/custom/pipx\venvs\") → False → falls through to "pip".

This is a real correctness bug, not just a test issue: any Windows user whose PIPX_HOME carries forward slashes (env files copied from a POSIX machine, WSL setups, scripted exports) would have pipx misdetected. The other tests in this class happened to pass on Windows only because they used forward-slash paths on both sides of the comparison and didn't go through the os.path.join branch.

Classification: Deterministic code regression (Windows path handling). Not flaky, not infrastructure.

Fix

Normalize both the executable path and each candidate pattern with os.path.normpath() before the prefix check. The same treatment is applied to the ~/.local/pipx/... fallback branch for consistency.

exe_path = os.path.normpath(sys.executable)
...
pattern = os.path.normpath(os.path.join(pipx_home, "venvs")) + os.sep
if exe_path.startswith(pattern):
    return "pipx"

This keeps the production logic minimal and platform-agnostic; the test was already exercising a realistic input shape and didn't need to change.

Verification

• ✅ 162 tests in tests/cli/test_doctor.py pass on Linux.
• ✅ Direct simulation under ntpath (Windows path semantics) confirms:
  • test_detect_pipx_via_pipx_home_env now returns "pipx" (was "pip").
  • All other 4 TestInstallMethodDetection cases still behave correctly.
• ✅ ruff check clean.
• ✅ Other matrix variants (Linux py3.11/3.12, macOS) were already green; no behavior change for forward-slash-on-both-sides cases.

Impact

• src/cli/doctor.py — _detect_install_method() only (used in 4 call sites within the same file for install/upgrade flows).
• No public API change.
• No test changes.

Trade-offs considered

• Fix in test instead (use platform-appropriate paths): rejected — the test input represents a realistic env-var shape and the production code should handle it. Project rule: "code correction over test weakening."
• Use pathlib.Path: rejected — os.path.startswith-style prefix matching is more straightforward than pathlib's is_relative_to, which would require try/except and adds an import.

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