fix(pi-extension): run script/executable PLANNOTATOR_BROWSER paths directly on macOS (#1429)

* fix(pi-extension): run script/executable PLANNOTATOR_BROWSER paths directly on macOS

open -a treats the value as an app bundle and fails with LaunchServices
-10811 for script paths, with the error swallowed by the detached spawn.
Mirror the binary's openBrowser branch: a value containing '/' that does
not end with '.app' is executed directly with the URL as its argument.

Fixes #1391

* test(pi-extension): pin the darwin PLANNOTATOR_BROWSER branch on non-darwin CI

The #1391 test reaches the darwin branch only on a real Mac; `bun test` runs
on ubuntu, where a configured browser falls through to the generic branch and
the regression is unguarded. Fake the platform the way the #1472 WSL tests
already do, and cover both arms: a slash-containing non-.app value is spawned
directly, while an app name or .app bundle still goes through `open -a`.

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Co-authored-by: PunkDevRobot <punkdevninja@proton.me>
Co-authored-by: Michael Ramos <mdramos8@gmail.com>