many tests patched sys.platform or a module's _IS_WINDOWS flag, then
ran on linux ci. the patch selects the branch under test, but the host
does not have the behavior the branch exists for. the test proves the
patch, not the platform. some gated assertions never ran on any host.
this commit adds three markers: linux_only, macos_only, windows_only.
a conftest hook skips a marked test on the other hosts, with a clear
reason. no test fakes a host now. two documented fakes remain
(android/termux, freebsd) because no ci runner exists for them.
each fake site got one of four treatments:
- gate it: the real host supplies the platform; mocks cover real
dependencies only, never host identity
- patch the module's own probe when the subject is the probe's consumer
- assert against the real host when the fake stood in for any non-x host
- delete the patch when it set the value the host already has
bare skipif(sys.platform != ...) guards became markers too. the lane
model skips these on linux and never imports them on windows, so they
ran on no host. platform parametrize tables are now one marked test
per os.
running on real hosts found real errors: a chrome-sandbox failure in
test_gui_command that main hides, and two windows failures fixed here.
the agents.md testing section now documents the policy.
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