fix(events): stop falling back to a fake "pwsh" argv when no launcher exists

[Noor-ul-ain001]

Summary

• _resolve_event_command_argv's ps branch did:

  launcher = shutil.which("pwsh") or shutil.which("powershell") or "pwsh"

  When neither pwsh nor powershell is on PATH, this silently returns ["pwsh", "-File", <script>, ...] instead of degrading to "no runnable script" (None) like every other failure branch in this same function (unparseable frontmatter, non-mapping scripts, unresolvable script path, etc.).
• subprocess.run() then raises FileNotFoundError trying to exec a binary that was just proven absent, which resolve_and_run_event_command's generic exception handler reports as a confusing Event command X error: [Errno 2] No such file or directory: 'pwsh' (exit code 2) instead of the clean No script found for event command warning (exit code 0) every other missing-script case gets.
• This function is explicitly documented as mirroring the generated dispatcher's own stdlib-only _resolve_argv (same file), which already gets this right: if not launcher: return None. The two had drifted apart.
• Fix: mirror that behavior — return None when neither launcher is found, instead of fabricating an argv naming a binary that doesn't exist.

Test plan

• Added test_ps_variant_returns_none_when_no_launcher_available to tests/integrations/test_events.py::TestCommandRunner: with shutil.which mocked to return None for every name, the resolver must return None rather than an argv naming a nonexistent pwsh.
• Verified the test fails without the fix (test-the-test): it asserted argv is None but got ['pwsh', '-File', '.../boot.ps1'] — reproducing the exact bug.
• Ran the full tests/integrations/test_events.py suite: 118 passed, 4 pre-existing Windows symlink-elevation failures (need admin rights, unrelated to this change), 1 skipped.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

https://claude.ai/code/session_01PJHJ2dHP2RVCNncHqN8Qm9