Story

Add admission control to validate MachineConfigPool osImageStream references against cluster OSImageStream.

Acceptance Criteria

• Create ValidatingAdmissionPolicy that validates MCP .spec.osImageStream.name references
• Ensure validation checks that the referenced stream name exists in cluster OSImageStream .status.availableStreams
• Policy should only validate when osImageStream field is populated
• Validation should apply to both CREATE and UPDATE operations on MachineConfigPool
• Add clear, user-friendly error message when validation fails
• Test that valid stream names are accepted
• Test that invalid/non-existent stream names are rejected at admission time