fix(desktop): probe py -3.12 launcher as fallback in _known_good_python()

[vivekchand]

Summary

Fixes the recurring no_distribution Windows field failure (issue #5859) where _known_good_python() only checked one hardcoded per-user LOCALAPPDATA path for Python 3.12, returning None immediately for system-wide installs — leaving bootstrap_python_version stuck at "3.11" across multiple consecutive days of telemetry.

Root cause: _known_good_python() probed exactly one path (%LOCALAPPDATA%\Programs\Python\Python312\python.exe). When Python 3.12 was installed system-wide (or via a corporate IT policy, or the user's own python.org installer in all-users mode), the path didn't exist, the function returned None, and _retry_on_known_good_python() aborted without a retry interpreter.

Fix: After the per-user path check, fall through to py -3.12. The py launcher with an explicit minor version pin finds Python 3.12 wherever it lives — per-user, system-wide, or another path — without risking the "newest interpreter" trap that bare py poses (bare py would return Python 3.14, the very interpreter whose wheels are missing).

Changes

• desktop/app.py: Extended _known_good_python() with a shutil.which("py") + py -3.12 fallback (per-user path is tried first; fallback runs only when that path is absent or fails the probe).
• tests/test_desktop_bootstrap_resilience.py: Added two new tests:
  • test_known_good_python_falls_back_to_py_launcher_for_system_install — asserts that a system-wide Python 3.12 (not at the LOCALAPPDATA path) is found via py -3.12.
  • test_known_good_python_skips_py_launcher_when_absent — asserts that when py is not on PATH, no subprocess is spawned and None is returned cleanly.

Test plan

• pytest tests/test_desktop_bootstrap_resilience.py — 65 passed (includes 2 new tests)
• AST parse of both modified files passes
• Manual Windows smoke: machine with Python 3.12 in system-wide location (not %LOCALAPPDATA%) — bootstrap should succeed and report bootstrap_python = 3.12

Closes #5859

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No-PRD: field-failure hotfix — directly traceable to auto-filed telemetry issue with a single, narrow code path change.

🤖 Generated with Claude Code

https://claude.ai/code/session_01QngcJH4PkF4SgSdHs1UoV8

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Generated by Claude Code

[vivekchand]

E2E Gate failure — not this PR's code.

The gate timed out after 3600 s because queue-priority.yml cancelled the queued CI run at ~16:29 UTC when a commit landed on main; the gate kept polling for status reports that never arrived. This is a CI infrastructure artefact, not a test failure from the diff.

Every check that actually ran before the cancellation passed:

• ✅ Syntax & Lint
• ✅ OSS golden path
• ✅ Cross-repo handoff (C4)
• ✅ No third-party asset loads
• ✅ PR cites the product record
• ✅ CodeQL

The cancelled checks (MOAT Keystone, pip install matrix, API Tests, E2E Browser, MOAT Verifier, Entitlement API, Store invariants, Wheel install) need a fresh run against this commit. Re-running now.

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Generated by Claude Code

[8090-software-factory]

⚠️ Drift Bot (ClawMetry): 1 potential drift finding(s)

1. Blueprint: Desktop Application Distribution

File: desktop/app.py:830

The blueprint specifies that _known_good_python() addresses the interpreter "by path, never by re-probing" using the py launcher. The PR now adds a fallback to use py -3.12 when the per-user path is absent or fails, which contradicts the blueprint's design principle to avoid any py launcher usage.
true

[8090-software-factory]

✅ Drift Bot (ClawMetry): no drift detected

Drift Bot analyzed the changed files against this project's blueprints and requirements and found no drift.

[8090-software-factory]

✅ Drift Bot (ClawMetry): no drift detected

Drift Bot analyzed the changed files against this project's blueprints and requirements and found no drift.

[vivekchand]

This branch has a merge conflict with main that cannot be auto-resolved (update_pull_request_branch returns "merge conflict between base and head").

The PR touches desktop/app.py and tests/test_desktop_bootstrap_resilience.py. Recent merges to main (notably #5588 — the routes/entitlement.py package split — and #5828) may have created the conflict. To unblock:

1. git fetch origin main
2. git checkout bot-fix/issue-5859-known-good-python-py-launcher-fallback
3. git merge origin/main
4. Resolve any conflicts, then git push

Once pushed, CI will re-run automatically.

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Generated by Claude Code

[8090-software-factory]

✅ Drift Bot (ClawMetry): no drift detected

Drift Bot analyzed the changed files against this project's blueprints and requirements and found no drift.