Story:

PR #5476 introduced basic status reporting for status.osImageStream in MachineConfigPools, which currently only copies the value from spec.osImageStream to the status. This means if a user hasn't explicitly set spec.osImageStream, the status field will be empty, providing no visibility into which OS image stream is actually being used.

We need to enhance this logic to always report the active OS image stream in the status, even when the user hasn't explicitly selected one. In such cases, the status should reflect the default stream from the OSImageStream CR's status.defaultStream field.

This improvement will give cluster administrators clear visibility into which OS image stream each pool is using, regardless of whether it was explicitly configured or defaulted.

Acceptance Criteria:

  - When spec.osImageStream is explicitly set by the user, status.osImageStream reports that value (existing behavior preserved)
  - When spec.osImageStream is empty/not set, status.osImageStream is populated with the default stream value from the OSImageStream CR's status.defaultStream field
  - The status.osImageStream field is always populated when the OSStreams feature gate is enabled, never left empty
  - Unit tests verify that:
    - Status correctly reflects explicit user-specified streams
    - Status correctly falls back to the default stream when spec is empty
    - Status updates properly when transitioning between explicit and default streams
  - e2e tests confirm that:
    - A new MCP with no spec.osImageStream shows the default stream in status
    - Existing MCPs without spec.osImageStream get updated to show the default stream
    - The reported stream matches the actual OS image being used by nodes in the pool