Create a helper function to poll for knowledge base database creation

[ommistry2914]

• Add wait_for_database_online helper methods to KnowledgeBaseResource and AsyncKnowledgeBaseResource so caller can wait for the knowledge base database to reach ONLINE without writing their own polling loops.

• New methods :

     - KnowledgeBasesResources.wait_for_database_online(uuid,*,timeout=600,interval=5,raise_on_failed=True)
     - AsyncKnowledgeBaseResource.wait_for_database_online(uuid,*,timeout=600,interval=5,raise_on_failed=True)
  

• Behavior

     - Polls retrieve(uuid) until database_status == "ONLINE"
     - Treats "UNHEALTHY" and "DECOMMISSIONED" as terminal failed states (raises RuntimeError when raise_on_failed=True).
     - Raises TimeoutError if timeout is exceeded.
     - Uses _sleep(interval) / await_sleep(interval) to avoid blocking.
  

• Example
  - Sync
  client = Gradient(...) kb = client.knowledge_bases.wait_for_database_online("kb-uuid", timeout=600, interval=5)

     - Async:
           `async_client = AsyncGradient(...)
            kb = await async_client.knowledge_bases.wait_for_database_online("kb-uuid", timeout=600, interval=5)`
  

fixes #42

[bbatha]

Closing as a dupe of #49