Standardize test seams for ROSA CLI commands

Summary

Standardize the testability pattern used by ROSA CLI command packages under cmd/ so future coverage work does not introduce multiple dependency-injection conventions.

cmd/initialize/cmd.go currently uses an explicit initDeps / defaultInitDeps() dependency bundle. Other commands generally use runWithRuntime(*rosa.Runtime) with runtime mocks, or package-level function-variable seams (for example, cmd/download/oc) that tests temporarily override and restore.

This follow-up is intentionally separate from the test-coverage work in #3368.

Rationale

A single, documented convention will reduce cognitive overhead and make command implementations and tests easier to maintain consistently.

Affected areas

• cmd/initialize/cmd.go
• Existing command packages that need seams not represented by *rosa.Runtime
• Command testing guidance and examples, as appropriate

Required changes

1. Agree on the preferred command test-seam pattern for dependencies outside *rosa.Runtime.
2. Refactor cmd/initialize to the agreed pattern, if appropriate.
3. Apply the agreed pattern to comparable command packages where doing so improves consistency.
4. Ensure tests that override package-level variables restore their original values to prevent cross-test interference.

Acceptance criteria

• A documented and consistently applied convention exists for command-level test seams.
• cmd/initialize follows that convention or has a documented exception.
• Relevant tests remain deterministic and do not leak modified global state.
• Targeted tests and the repository's standard Go verification commands pass.

Context

Requested by @amandahla.

• Pull request: https://github.com/openshift/rosa/pull/3368
• Discussion: https://github.com/openshift/rosa/pull/3368#discussion_r3595743058