Move webhooks out of exp/api/v1beta1

[mboersma]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Mirrors #6141 for the exp/api/v1beta1 tree. Webhook code (validation + defaulting) and the supporting default-helpers move out of the API types package into purpose-built internal packages, leaving exp/api/v1beta1 to hold types and conversions only.

• New internal/exp/api/v1beta1/ holds the Set* defaulters as free functions taking *infrav1exp.AzureMachinePool (was receiver methods on the type).
• New internal/exp/webhooks/ holds the AzureMachinePoolWebhook and AzureMachinePoolMachineWebhook types plus the per-validator helpers.
• ValidateAzureMachinePool(old, amp, client) is exported so the existing external test in exp/api/v1beta1/azuremachinepool_test.go (package v1beta1_test) can keep covering the validators directly.
• main.go registers the webhooks from the new package.
• The Makefile adds paths=./internal/exp/webhooks/... to the generate-manifests controller-gen invocation. config/webhook/manifests.yaml is byte-identical before and after — the same kubebuilder markers in their new home produce the same admission config.
• exp/controllers/azuremachinepool_reconciler.go now calls apiinternalexp.SetNetworkInterfacesDefaults(s.scope.AzureMachinePool) instead of the receiver method.
• Deletes the original exp/api/v1beta1/azuremachinepool_default.go, azuremachinepool_default_test.go, azuremachinepool_webhook.go, azuremachinepool_webhook_test.go, azuremachinepoolmachine_webhook.go. Git detected these as renames.

//nolint:staticcheck annotations on accesses to deprecated RoleAssignmentName, SubnetName, and AcceleratedNetworking mirror what #6141 added in internal/api/v1beta1/azuremachine_default.go — they're necessary because reading those fields from outside their defining package now triggers SA1019.

Which issue(s) this PR fixes:
Fixes #6167

Special notes for your reviewer:

The diff looks large but the bulk is mechanical: methods on *AzureMachinePool become free functions, and tests update their call-sites accordingly. The only behavioral change is the new package layout — all admission webhook semantics are unchanged (verified by the byte-identical config/webhook/manifests.yaml).

TODOs:

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

Release note:

NONE

[codecov]

Codecov Report

❌ Patch coverage is 69.56522% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.84%. Comparing base (bd0f01b) to head (0e3eea6).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
...ternal/exp/api/v1beta1/azuremachinepool_default.go	70.83%	7 Missing ⚠️
internal/exp/webhooks/azuremachinepool_webhook.go	81.08%	6 Missing and 1 partial ⚠️
...al/exp/webhooks/azuremachinepoolmachine_webhook.go	0.00%	5 Missing ⚠️
main.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6280      +/-   ##
==========================================
- Coverage   43.95%   43.84%   -0.11%     
==========================================
  Files         289      289              
  Lines       25351    25346       -5     
==========================================
- Hits        11143    11114      -29     
- Misses      13435    13458      +23     
- Partials      773      774       +1     

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[willie-yao]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Details

Git tree hash: 317aa06247659a98142013b4a7184bbf6fc72f09

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[willie-yao]

/retest

[k8s-ci-robot]

@mboersma: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-provider-azure-e2e	0e3eea6	link	unknown	/test pull-cluster-api-provider-azure-e2e
pull-cluster-api-provider-azure-e2e-workload-upgrade	0e3eea6	link	unknown	/test pull-cluster-api-provider-azure-e2e-workload-upgrade
pull-cluster-api-provider-azure-apidiff	0e3eea6	link	false	/test pull-cluster-api-provider-azure-apidiff

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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