Preserve image plan metadata in VMSS model conversion to prevent false model drift

[stelucz]

What type of PR is this?

/kind bug

What this PR does / why we need it:

This PR fixes a VMSS model comparison issue in CAPZ where plan metadata was lost during Azure SDK → CAPZ image conversion.
As a result, CAPZ could detect a model mismatch even when AzureMachinePool spec was unchanged, and repeatedly roll MachinePool instances.

Root Cause

• LatestModelApplied for AzureMachinePoolMachine depends on comparing:
  • desired image from MachinePoolScope.GetVMImage(...)
  • observed VM/VMSS instance image from Azure APIs.
• For Compute/Shared Gallery images that include a marketplace-like plan, CAPZ expected plan data in desired model.
• Before this change, converter logic SDKImageToImage did not carry Plan fields for gallery-based image references.
• This produced structurally different image objects (desired had plan metadata, observed often did not), leading to persistent LatestModelApplied=false.

Actual State Before Change

• No user-visible spec mutation on MachinePool/AzureMachinePool.
• CAPZ logs repeatedly showed model-change predicates firing and ongoing reconciles/deletes:
  • MachinePoolModelHasChanged ... shouldUpdate=true
  • repeated Deleting AzureMachinePoolMachine ...
• Node/VM churn continued despite stable target configuration.

Change Introduced

• Added plan-aware conversion path in VMSS converter:
  • SDKImageToImageWithPlan(imageRef, isThirdParty, sdkPlan)
  • Existing SDKImageToImage(imageRef, isThirdParty) preserved as compatibility wrapper.
• Updated VM/VMSS conversion call sites to pass both:
  • existing isThirdParty signal (sdkPlan != nil)
  • full sdkPlan object when available.
• Added plan propagation for gallery images:
  • Compute Gallery: map Plan into ComputeGallery.Plan
  • Shared Gallery: map Plan into Publisher/Offer/SKU fields
• Updated/extended unit tests in converter package to cover the plan-carrying behavior.

Expected Result

• Observed image model now preserves relevant plan metadata, reducing false differences against desired model.
• LatestModelApplied is evaluated against a more faithful observed state.
• Prevents unnecessary VMSS instance replacement loops caused by plan metadata loss.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• cherry-pick candidate

Release note:

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[stelucz]

/assign vincepri

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.47%. Comparing base (012443d) to head (c380ed5).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6129      +/-   ##
==========================================
+ Coverage   44.41%   44.47%   +0.05%     
==========================================
  Files         280      280              
  Lines       25379    25381       +2     
==========================================
+ Hits        11273    11289      +16     
+ Misses      13293    13281      -12     
+ Partials      813      811       -2     

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[stelucz]

added missing test