fix: keep bootstrapRef across the VM creation request-id patch

[vijayaraghavanr31]

Summary

• The VM creation request-id patch dropped an in-memory spec.bootstrapRef, because controller-runtime zeros the object before decoding the API response.
• addGuestCustomizationToVM then panicked on bootstrapRef.Kind during the first VM create. The retry usually succeeded, which is why scale tests logged the panic and the cluster often still came up.
• The request-id patch now persists bootstrapRef, so cloud-init is still applied on the first create. Both guest-customization helpers return an error when the ref is nil instead of panicking.

Test plan

• go test -count=1 -run 'TestNutanixMachineReconciler_getOrMintVMCreationRequestID|TestAddGuestCustomizationNilBootstrapRef|TestNutanixMachineReconciler_getOrCreateVM' ./controllers/
• Create a cluster and confirm CAPX logs have no addGuestCustomizationToVM nil-pointer panic on the first VM create
• Confirm the created VM has guest customization and the node joins

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.52%. Comparing base (883f3c9) to head (9c3baed).
⚠️ Report is 131 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #800       +/-   ##
===========================================
+ Coverage   43.96%   54.52%   +10.56%     
===========================================
  Files          18       32       +14     
  Lines        2593     6294     +3701     
===========================================
+ Hits         1140     3432     +2292     
- Misses       1417     2546     +1129     
- Partials       36      316      +280     

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