fix(gateway): widen cron namespace-collision fix to all migrated adapters

#49431 corrected parents[2]->parents[3] for discord + raft only. The same
bug existed in slack, whatsapp, and telegram adapters (migrated from
gateway/platforms/ in 5600105478): each inserts parents[2] = plugins/ onto
sys.path[0], shadowing the real cron/ package with plugins/cron/ so
'import cron.scheduler_provider' raises ModuleNotFoundError on gateway start.

Fixes #49410, #49824.

plugins/platforms/slack/adapter.py

@@ -34,7 +34,7 @@ except ImportError:
 import sys
 from pathlib import Path as _Path
 
-sys.path.insert(0, str(_Path(__file__).resolve().parents[2]))
+sys.path.insert(0, str(_Path(__file__).resolve().parents[3]))
 
 from gateway.config import Platform, PlatformConfig
 from gateway.platforms.helpers import MessageDeduplicator

plugins/platforms/telegram/adapter.py

@@ -63,7 +63,7 @@ except ImportError:
 
 import sys
 from pathlib import Path as _Path
-sys.path.insert(0, str(_Path(__file__).resolve().parents[2]))
+sys.path.insert(0, str(_Path(__file__).resolve().parents[3]))
 
 from gateway.config import Platform, PlatformConfig
 from gateway.platforms.base import (

plugins/platforms/whatsapp/adapter.py

@@ -178,7 +178,7 @@ def _terminate_bridge_process(proc, *, force: bool = False) -> None:
         return
 
 import sys
-sys.path.insert(0, str(Path(__file__).resolve().parents[2]))
+sys.path.insert(0, str(Path(__file__).resolve().parents[3]))
 
 from gateway.config import Platform, PlatformConfig
 from gateway.platforms.whatsapp_common import WhatsAppBehaviorMixin