refactor: refactor mtls test to support Istio 1.27+ native sidecars

[emmaaroche]

Description

• Refactored mTLS test test_pods_have_istio_sidecar_and_labels to support Istio 1.27+ native sidecars which now default to initContainers instead of regular containers
• Maintained backward compatibility with Istio <1.27 by checking both container locations

Closes #882

Changes

• Updated test_pods_have_istio_sidecar_and_labels test to verify istio-proxy presence in both container locations
• Updated wait_for_injected_pod fixture in conftest.py to check both spec.containers and spec.initContainers for the istio-proxy sidecar
• Added or [] fallback when accessing initContainers as it's optional in the pod spec

Verification steps

Run the test on a cluster with Istio 1.27 (I tested on kua-420-2 which is configured with it):

 poetry run pytest -vv testsuite/tests/singlecluster/gateway/mtls/test_mtls_configuration.py::test_pods_have_istio_sidecar_and_labels

[silvi-t]

LGTM

[azgabur]

Tests passed, otherwise LGTM