idf-ci emits a skip pipeline containing only a fake_pass job, with no
include of test_child_pipeline.yml, when the modified files match no
buildable apps. patch_buildv2_child_pipeline.py redefined
generate_pytest_child_pipeline unconditionally, assuming the included
template supplied its tags, image and assign_test stage via keyword
merge. With the template absent, the injected job was bare: no tags,
default test stage. It was scheduled on the wrong runner and failed,
turning the allow_failure build_child_pipeline_buildv2 bridge red on
every buildv2 MR whose changes produce an empty app matrix.
Return early when a fake_pass job is present so the child pipeline is
left untouched and skips, matching the default build path. Add a unit
test for the patch script and a host_test job to run it.
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