Add os_disk_caching and data_disk_caching builder operations for VM/VMSS

[Copilot]

Exposes the caching property for OS and data disks on VMs and VM Scale Sets, allowing control over host-level disk caching behavior.

The changes in this PR are as follows:

• Common.fs: Added CachingType DU (NoCaching → "None", ReadOnly, ReadWrite) and Caching: CachingType option field to DiskInfo
• Builders.Vm.fs: Added os_disk_caching and data_disk_caching builder operations; caching is applied in Run composably alongside disk_delete_option; only applies to image-based OS disks and empty data disks (not attached disks)
• Compute.fs: storageProfile now emits caching in ARM output when set; omitted (not null-emitted) when unset, preserving Azure's default behaviour
• Tests: 5 new tests covering OS caching, data disk caching, NoCaching ARM value, absence when unset, and composition with disk_delete_option

I have read the contributing guidelines and have completed the following:

• Tested my code end-to-end against a live Azure subscription.
• Updated the documentation in the docs folder for the affected changes.
• Written unit tests against the modified code that I have made.
• Updated the release notes with a new entry for this PR.
• Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here:

• End-to-end testing against a live Azure subscription was not performed in this automated implementation.

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

open Farmer
open Farmer.Builders
open Farmer.Vm

let myVm = vm {
    name "my-vm"
    username "azureuser"
    vm_size Standard_D2s_v3
    operating_system UbuntuServer_2204LTS
    os_disk 128 Premium_LRS
    os_disk_caching ReadWrite        // Set OS disk host caching
    add_disk 512 Premium_LRS
    data_disk_caching ReadOnly       // Set caching for all data disks
}

let deployment = arm {
    location Location.EastUS
    add_resource myVm
}

[ninjarobot]

New features look right and created the VM and VMSS disks with the correct settings. Regression tested. Approved.