refactor: make all integration test files self-contained

Each test file now creates and cleans up its own resources:
- 06_VMs: restores own AfterAll cleanup for pester-test-vm
- 07_Snapshots: creates pester-snap-vm, tests snapshots, cleans up
- 09_CloudInit: creates pester-ci-vm with cloud-init drive, cleans up
- 15_Tasks: creates pester-task-vm, tests task CRUD, cleans up

Removed cross-file dependency helpers (Find-TestVm, Register-TestResource)
from _IntegrationHelper.ps1 — no longer needed.

Each file can now run independently via -Tests filter. If a Setup
context fails, subsequent tests in the same file skip gracefully.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/PSProxmoxVE.Tests/Integration/11_LinuxVM.Tests.ps1

@@ -62,7 +62,6 @@ Describe 'Linux VM — Integration' -Tag 'Integration' {
 
             $script:LinuxVmId = $vm.VmId
             $script:CreatedVmIds.Add($vm.VmId)
-            Register-TestResource -Name 'pester-linux-vm' -VmId $vm.VmId
         }
 
         It 'Should import cloud image disk (Import-PveVmDisk)' {