ci(buildv2): skip patching the fake_pass build child pipeline

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.

tools/ci/dynamic_pipelines/scripts/patch_buildv2_child_pipeline.py

@@ -33,6 +33,13 @@ def patch(path: str) -> None:
     with open(path) as f:
         d = yaml.safe_load(f)
 
+    if 'fake_pass' in d:
+        # idf-ci emitted a skip pipeline (no apps matched the modified files): there
+        # are no build jobs to activate and no included generate_pytest_child_pipeline
+        # to redefine. Leave it untouched so Build system v2 skips like the default path.
+        print('Skip pipeline (fake_pass) detected, leaving it untouched')
+        return
+
     injected = []
     for k, v in d.items():
         if isinstance(v, dict) and 'extends' in v: