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.

.gitlab/ci/host-test.yml

@@ -295,6 +295,12 @@ test_idf_build_apps_load_soc_caps:
     - cd tools/ci
     - pytest_for_ut ./test_soc_headers_load_in_idf_build_apps.py
 
+test_dynamic_pipelines_scripts:
+  extends: .host_test_template
+  script:
+    - cd ${IDF_PATH}/tools/ci/dynamic_pipelines/tests
+    - pytest_for_ut ./test_patch_buildv2_child_pipeline.py
+
 test_nvs_gen_check:
   extends: .host_test_template
   artifacts:

.gitlab/ci/rules.yml

@@ -134,6 +134,9 @@
 
   - "tools/ci/test_soc_headers_load_in_idf_build_apps.py"
 
+  - "tools/ci/dynamic_pipelines/scripts/patch_buildv2_child_pipeline.py"
+  - "tools/ci/dynamic_pipelines/tests/**/*"
+
 .patterns-docker: &patterns-docker
   - "tools/docker/**/*"