[ND 4.2+] Add Fabric Software Prepare/Stage Module (nd_fabric_prepare_update)

[sivakasi-cisco]

Summary

Add nd_fabric_prepare_update to support the Nexus Dashboard 4.2 Fabric Software Management Prepare stage (softwareUpdatePlan/actions/stage).

Related PR

• nd_fabric_prepare_update - ND 4.2 software staging #291

Scope

• Stage and validate the configured image for one or more update groups.
• Reject mixed-role update groups before staging.
• Remain idempotent when update groups are already staged.
• Support optional completion polling with wait: true.
• Add the required endpoint models, orchestrator logic, documentation, unit tests, and integration tests.

Acceptance Criteria

• Mixed-role groups are rejected before an update is started.
• Already-staged groups report no change.
• wait: true polls safely to completion and tolerates transient transport errors.
• Sanity, unit, and live ND 4.2.1 integration tests pass.

Out of Scope

• Image upload, software installation, and activation.