fix: correct fake WireGuard key length and guard cell2_client teardown

The synthetic cell fixture used a 46-char base64 key where the validator
expects exactly 43 chars before '='. The key failed format validation so
add_cell_peer returned False, making the cell connection store nothing and
all TestCellPermissionsApi tests hit 404.

The TestCellServiceAccessRestrictions and TestLiveCellConnection teardown
fixtures called _remove_connection(cell2_client, ...) without checking if
cell2_client is None (expected when no second cell is configured), causing
AttributeError on teardown.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/e2e/api/test_cell_to_cell.py

@@ -193,7 +193,7 @@ class TestCellPermissionsApi:
         fake_dns_ip = '10.99.0.1'
         fake_invite = {
             'cell_name': 'e2etest-synthetic-cell',
-            'public_key': 'AAAAFakePublicKeyForE2eTestingAAAAAAAAAAAAAAAA=',
+            'public_key': 'FakePublicKeyForE2eCellTestAAAAAAAAAAAAAAAA=',
             'endpoint': '127.0.0.2:51820',
             'vpn_subnet': fake_subnet,
             'dns_ip': fake_dns_ip,
@@ -334,7 +334,7 @@ class TestLiveCellConnection:
 
         if cell2_name:
             _remove_connection(admin_client, cell2_name)
-        if cell1_name:
+        if cell1_name and cell2_client:
             _remove_connection(cell2_client, cell1_name)
 
     def _connect_cells(self, admin_client, cell2_client,
@@ -530,7 +530,7 @@ class TestCellServiceAccessRestrictions:
             cell1_name = None
         if cell2_name:
             _remove_connection(admin_client, cell2_name)
-        if cell1_name:
+        if cell1_name and cell2_client:
             _remove_connection(cell2_client, cell1_name)
 
     def _get_forward_rules(self, client) -> str: