chore: migrate samples to google-cloud-python system test infrastructure

[ohmayr]

This PR migrates the sample testing infrastructure away from the broken/legacy python-docs-samples secrets setup. It adopts the standard python-multi docker image and trampoline.sh execution flow used by the rest of google-cloud-python. Additionally, it permanently drops the .kokoro configurations and required GitHub status checks for the unsupported Python 3.7, 3.8, and 3.9 environments.

Fixes: #2767

[gemini-code-assist]

Code Review

This pull request migrates the samples directory to tests/system and updates the corresponding Kokoro configuration files, test scripts, Makefile, and other tooling to reference the new path. The review feedback highlights a critical issue where periodic builds rewinding to older releases will fail because tests/system does not exist yet; a dynamic fallback to samples is suggested. Additionally, the reviewer recommends updating TRAMPOLINE_BUILD_FILE in periodic-head.cfg files to reflect the renamed script, and adding safety checks to ensure KOKORO_GFILE_DIR is non-empty before checking for files within it.

[gemini-code-assist]

During periodic builds, the repository is checked out at the LATEST_RELEASE tag (which was created before this migration). At that tag, the tests/system directory does not exist, but samples does. As a result, find tests/system will fail, and the script will silently exit with 0 without running any tests.

To prevent this, we should dynamically fall back to samples if tests/system is not present.

Suggested change

	# Exit early if samples don't exist
	if ! find tests/system -name 'requirements.txt' | grep -q .; then
	echo "No tests run. './tests/system/**/requirements.txt' not found"
	exit 0
	fi
	# Exit early if tests don't exist
	TEST_DIR="tests/system"
	if [ ! -d "$TEST_DIR" ] && [ -d "samples" ]; then
	TEST_DIR="samples"
	fi
	if ! find "$TEST_DIR" -name 'requirements.txt' | grep -q .; then
	echo "No tests run. './$TEST_DIR/**/requirements.txt' not found"
	exit 0
	fi

[gemini-code-assist]

Use the dynamically resolved $TEST_DIR variable to support running tests on older releases during periodic builds.

Suggested change

	for file in tests/system/**/requirements.txt; do
	for file in "$TEST_DIR"/**/requirements.txt; do

[gemini-code-assist]

Since .kokoro/test-samples-against-head.sh has been renamed to .kokoro/test-system-against-head.sh, please make sure to update the TRAMPOLINE_BUILD_FILE environment variable in all periodic-head.cfg files (e.g., .kokoro/system/python3.10/periodic-head.cfg, etc.) to point to the new filename. Currently, they still reference the old .kokoro/test-samples-against-head.sh path, which will cause those builds to fail.

[gemini-code-assist]

It is safer to ensure KOKORO_GFILE_DIR is non-empty before checking for the existence of project-id.json to avoid checking the root directory /project-id.json if the variable is unset.

Suggested change

	if [[ -f "${KOKORO_GFILE_DIR}/project-id.json" ]]; then
	export PROJECT_ID=$(cat "${KOKORO_GFILE_DIR}/project-id.json")
	export GOOGLE_CLOUD_PROJECT="${PROJECT_ID}"
	gcloud config set project "$PROJECT_ID"
	fi
	if [[ -n "${KOKORO_GFILE_DIR}" && -f "${KOKORO_GFILE_DIR}/project-id.json" ]]; then
	export PROJECT_ID=$(cat "${KOKORO_GFILE_DIR}/project-id.json")
	export GOOGLE_CLOUD_PROJECT="${PROJECT_ID}"
	gcloud config set project "$PROJECT_ID"
	fi

[gemini-code-assist]

It is safer to ensure KOKORO_GFILE_DIR is non-empty before checking for the existence of service-account.json to avoid checking the root directory /service-account.json if the variable is unset.

Suggested change

	if [[ -f "${KOKORO_GFILE_DIR}/service-account.json" ]]; then
	export GOOGLE_APPLICATION_CREDENTIALS="${KOKORO_GFILE_DIR}/service-account.json"
	gcloud auth activate-service-account --key-file="${GOOGLE_APPLICATION_CREDENTIALS}"
	fi
	if [[ -n "${KOKORO_GFILE_DIR}" && -f "${KOKORO_GFILE_DIR}/service-account.json" ]]; then
	export GOOGLE_APPLICATION_CREDENTIALS="${KOKORO_GFILE_DIR}/service-account.json"
	gcloud auth activate-service-account --key-file="${GOOGLE_APPLICATION_CREDENTIALS}"
	fi

[daniel-sanche]

LGTM