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/_IntegrationHelper.ps1

@@ -145,43 +145,3 @@ function script:Disconnect-TestPve {
     #>
     try { Disconnect-PveServer -ErrorAction SilentlyContinue } catch { }
 }
-
-# ── Resource discovery helpers ───────────────────────────────────────────
-# These allow later test files to find resources created by earlier files
-# without sharing script-scoped variables. If the resource doesn't exist,
-# the caller gets $null and should skip.
-
-function script:Find-TestVm {
-    <#
-    .SYNOPSIS
-        Finds the pester-test-vm by checking env state or querying PVE.
-    #>
-    param([string]$Name = 'pester-test-vm')
-
-    # Check env var first (set by the file that created it)
-    $envKey = "PVETEST_STATE_$($Name.Replace('-','_').ToUpper())"
-    $vmId = [System.Environment]::GetEnvironmentVariable($envKey)
-    if ($vmId) { return [int]$vmId }
-
-    # Fall back to querying PVE
-    try {
-        $vms = Get-PveVm -Node $script:Node -ErrorAction Stop
-        $match = $vms | Where-Object { $_.Name -eq $Name } | Select-Object -First 1
-        if ($match) { return [int]$match.VmId }
-    } catch { }
-
-    return $null
-}
-
-function script:Register-TestResource {
-    <#
-    .SYNOPSIS
-        Registers a resource VMID in env for cross-file discovery.
-    #>
-    param(
-        [string]$Name,
-        [int]$VmId
-    )
-    $envKey = "PVETEST_STATE_$($Name.Replace('-','_').ToUpper())"
-    [System.Environment]::SetEnvironmentVariable($envKey, $VmId.ToString())
-}