test(conformance): port the credential gating e2e tests #5317

Description

Port the six credential gating tests from e2e/rust/tests/credential_gating.rs (added in #5264) into the conformance family, then remove the old test and its Podman CI entry.

This is the pilot port that #5296 asks for. It records which testkit helpers the port needed, without adding the full helper inventory.

Acceptance

• The six tests run as credential_gating::* in openshell-conformance and keep the assertions from test(e2e): split credential_gating into independent parallel tests #5264.
• Each test isolates its provider, profile and sandbox names, and cleans them up when it fails.
• The Docker, Podman, and k3s lanes run the suite.
• e2e/rust/tests/credential_gating.rs, its [[test]] entry, and its PODMAN_CI_TESTS entry are removed.

Part of #5281. Relates to #5296.