Async watcher Subscribe call is missing auth metadata

[zeevdr]

Description

sdk/src/opendecree/async_watcher.py:209-213 invokes Subscribe without passing metadata=self._metadata(). AsyncConfigClient uses a raw (un-intercepted) channel and injects metadata on every unary call; the async watcher bypasses this. Auth-enabled servers reject the Subscribe with UNAUTHENTICATED.

Acceptance criteria

• Thread metadata through AsyncConfigWatcher
• Pass metadata=... to Subscribe
• Add an integration test against a real server with auth enabled