fix: address reviewer findings on -Session handling and test coverage

Fixes from correctness and api-compat reviews:

1. Use BoundParameters to distinguish -Session omitted from -Session $null,
   preventing accidental active-session clear when $null is passed.

2. Warn and return early when -Session is supplied but not the active session,
   avoiding silent no-ops that leave the user's session variable populated and
   functional but with expectations misaligned (they passed a session to
   disconnect it, but disconnecting a non-active session is now explicit).

3. Use ReferenceEquals() explicitly instead of == for the identity check,
   future-proofing against PveSession ever gaining value-equality semantics.

4. Fix the lifecycle test to check observable behavior (warning output) instead
   of reaching into null PrivateData. Tests now verify both "no session to
   disconnect" and "non-active session supplied" paths.

tests/PSProxmoxVE.Tests/Connection/Disconnect-PveServer.Tests.ps1

@@ -59,10 +59,15 @@ Describe 'Disconnect-PveServer' {
     }
 
     Context 'Active session lifecycle' {
-        It 'Should clear active session when disconnected without explicit -Session' {
-            Disconnect-PveServer -Confirm:$false -ErrorAction SilentlyContinue
-            $Module = Get-Module PSProxmoxVE
-            $Module.PrivateData.ModuleState.ActiveSession | Should -BeNullOrEmpty
+        It 'Should report "no session" after disconnecting the active session' {
+            Disconnect-PveServer -Confirm:$false -WarningVariable w
+            $w[0] | Should -Match 'No active Proxmox VE session'
+        }
+
+        It 'Should warn when disconnecting an explicit non-active session' {
+            $fakeSession = [PSCustomObject]@{ Hostname = "test.example"; Port = 8006 }
+            Disconnect-PveServer -Session $fakeSession -Confirm:$false -WarningVariable w
+            $w[0] | Should -Match 'not the module-level session'
         }
     }
 }