[LogsDB] [Stack 8.19.21-SNAPSHOT] [network_traffic] Failing test daily: system test: cassandra-insert in network_traffic.cassandra

- Stack version: 8.19.21-SNAPSHOT
- LogsDB: enabled
- Package: network_traffic
- Failing test: system test: cassandra-insert
- DataStream: cassandra
- Owners:
- @elastic/security-service-integrations

Error:
```
could not assign policy to agent: error occurred while waiting for the policy to be assigned to all agents: can't get the agent: could not list agents: could not send request to Kibana API: Get "https://127.0.0.1:5601/api/fleet/agents/c98eebd7-e6ee-4be3-ab11-dc327838d8cc": context deadline exceeded
```

First build failed: https://buildkite.com/elastic/integrations/builds/49243

Start with the network_traffic system test named cassandra-insert and reproduce the failure against the LogsDB-enabled 8.19.21-SNAPSHOT stack. Trace the Kibana API request while assigning the policy to the Fleet agent, then verify the test completes reliably without the reported timeout.

Written by the indexing model from the issue text.