fix: validate API server LB private IP is unchanged on update

[dee077]

What type of PR is this?
/kind bug

What this PR does / why we need it:

• Fixes a bug where the API server load balancer private IP can be changed after AzureCluster creation when APIServerILB is enabled and the LB type is Public
• Previously, the validation only ran for Internal type LBs, leaving the Public + APIServerILB path unguarded
• Adds a function validateAPIServerLBPrivateIPUnchanged and uses it for both Internal and Public + APIServerILB paths, deduplicating the validation logic
• Fixes the error field path for the Internal LB immutability check from apiServerLB.name to apiServerLB.frontendIPConfigs[0].privateIP

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 #5314

Special notes for your reviewer:

TODOs:

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

Release note:

fix: validate API server LB private IP is unchanged on update for Public and APIServerILB configuration

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: dee077 / name: dee077 (944f04c)

[kubernetes-prow]

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[kubernetes-prow]

Hi @dee077. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[bryan-cox]

/ok-to-test

[bryan-cox]

Thanks for the contribution @dee077! Overall this is a solid bug fix — the core logic is correct, the deduplication via validateAPIServerLBPrivateIPUnchanged is well-motivated, and the corrected field path is a nice improvement over the original apiServerLB.name. A few items below to consider.

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 44.08%. Comparing base (47cc48d) to head (944f04c).

Files with missing lines	Patch %	Lines
internal/webhooks/azurecluster_validation.go	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6389      +/-   ##
==========================================
+ Coverage   44.06%   44.08%   +0.02%     
==========================================
  Files         289      289              
  Lines       25383    25392       +9     
==========================================
+ Hits        11185    11195      +10     
  Misses      13422    13422              
+ Partials      776      775       -1     

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[bryan-cox]

/lgtm

[kubernetes-prow]

LGTM label has been added.

Details

Git tree hash: fed77635399a776d0a2188fed9c36dcddff7a59a

[kubernetes-prow]

New changes are detected. LGTM label has been removed.

[willie-yao]

/approve

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: willie-yao

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Details

Needs approval from an approver in each of these files:

• OWNERS [willie-yao]

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[willie-yao]

/retest

[willie-yao]

/retest