[Az.Compute] Remove AFEC check for UseStandardSecurityType; Standard SecurityType now first-class

[Copilot]

CRP API 2025-11-01 promotes Standard to a first-class securityType enum — no AFEC registration required. The API also now returns "securityProfile": {"securityType": "Standard"} in all GET responses by default (previously returned null/empty).

Changes

Test assertions (VirtualMachineTests.ps1, VirtualMachineScaleSetTests.ps1)

• Replaced Assert-Null $vm.SecurityProfile → Assert-AreEqual $vm.SecurityProfile.SecurityType "Standard" across Test-VirtualMachineSecurityTypeStandard, Test-VirtualMachineSecurityTypeStandardWithConfig, Test-VirtualMachineScaleSetSecurityTypeStandard, Test-VirtualMachineScaleSetSecurityTypeStandardWithConfig, and the Standard-update branches of Test-VirtualMachineSecurityTypeWithoutConfig / Test-VirtualMachineScaleSetSecurityType
• Removed duplicate Assert-Null lines and stale comments referencing the old API limitation

Session recordings (6 files)

• Injected "securityProfile": {"securityType": "Standard"} into GET (and relevant PUT/PATCH) response bodies for all Standard-security VMs and VMSSes
• Replaced empty "securityProfile": {} with {"securityType": "Standard"} in responses after an Update-to-Standard operation

Stale code comment (VirtualMachineScaleSetCreateOrUpdateMethod.cs)

• Replaced misleading comment "API does not currently support Standard securityType value, so need to null it out here" — the code was already keeping "Standard" and merely normalizing its casing

Changelog

• Added entry to src/Compute/Compute/ChangeLog.md under ## Upcoming Release

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