Add a helper to select an available channel for an OpenShift version

Summary

Add a reusable test helper that takes an OpenShift version identifier and obtains the version's availableChannels from the OCM endpoint:

/api/clusters_mgmt/v1/versions/{version id}

Use the helper where E2E tests need a channel for a selected version.

Rationale

Tests must not hard-code a channel such as eus-4.16. The channels available for a version can change. The helper must derive a valid channel from the current OCM version metadata.

The current test can omit --channel. This follow-up provides the reusable helper for tests that must explicitly select a channel.

Affected area

• tests/e2e/test_rosacli_cluster.go
• Test utilities that access OCM version metadata

Acceptance criteria

• The helper accepts an OpenShift version identifier.
• The helper queries /api/clusters_mgmt/v1/versions/{version id}.
• The helper returns channel data from availableChannels.
• The helper handles missing or empty availableChannels with a clear error.
• E2E tests that require an explicit channel use the helper instead of a hard-coded channel value.
• Add focused test coverage for the helper.

References

• Pull request: https://github.com/openshift/rosa/pull/3472
• Review discussion: https://github.com/openshift/rosa/pull/3472#discussion_r3798173327
• Pull request review: https://github.com/openshift/rosa/pull/3472#pullrequestreview-4953820125
• Requested by: @amandahla