Add unit tests for azuremachine_validation

[mboersma]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Adds the missing unit-test coverage for internal/webhooks/azuremachine_validation.go called out in the linked issue:

• TestAzureMachine_ValidateDataDisks gains cases for disk.Lun nil and out-of-range (< 0, > 63).
• New TestAzureMachine_ValidateDiagnostics covers all branches of ValidateDiagnostics (nil diagnostics, nil boot, the three BootDiagnosticsStorageAccountType values, and missing/empty UserManaged.StorageAccountURI).

After this change, both ValidateDataDisks and ValidateDiagnostics report 100% statement coverage.

The ValidateDataDisksUpdate / validateManagedDisksUpdate cases mentioned in the issue are no longer applicable — those functions were removed during the webhook refactor.

Which issue(s) this PR fixes:
Fixes #3374

Special notes for your reviewer:

None.

TODOs:

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

Release note:

NONE

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.95%. Comparing base (bd1e799) to head (7410779).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6277      +/-   ##
==========================================
+ Coverage   43.88%   43.95%   +0.07%     
==========================================
  Files         289      289              
  Lines       25351    25351              
==========================================
+ Hits        11125    11143      +18     
+ Misses      13448    13435      -13     
+ Partials      778      773       -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[mboersma]

/retest

[willie-yao]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Details

Git tree hash: 77e534589aa1e915c20cb8b841abc410f5271b8c

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [willie-yao]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment