velero: surface malformed collector JSON and agent pod logs on failure

replay_collector_output called json.loads unguarded, so a malformed
collector blob surfaced as a bare JSONDecodeError with no indication of
which candidate produced it. run_discovery_check_kubernetes's failure
path also only showed the agent check invocation's own stdout/stderr,
not the agent pod's own logs, which is often where the real failure
(a crashed IPC/auth setup) shows up. Mirror dd_agent_check's handling
of both cases.

Environment: Datadog workspace

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: vitkyrka

datadog_checks_dev/datadog_checks/dev/kube_discovery.py

@@ -157,7 +157,10 @@ def run_discovery_check_kubernetes(
 
     result = exec_agent_pod(state['kubeconfig_path'], state['namespace'], command, capture=True, check=False)
     if not replay_collector_output(result.stdout, aggregator, datadog_agent):
-        raise ValueError(f'Could not find valid check output:\n{result.stdout}\n{result.stderr}')
+        agent_logs = get_pod_logs(state['kubeconfig_path'], state['namespace'], AGENT_POD_NAME)
+        raise ValueError(
+            f'Could not find valid check output:\n{result.stdout}\n{result.stderr}\n\nAgent pod logs:\n{agent_logs}'
+        )
 
     return aggregator
 
@@ -228,7 +231,10 @@ def replay_collector_output(output: str, aggregator: Any, datadog_agent: Any) ->
     """Replay collector JSON blobs from ``agent check --json`` output."""
     matches = find_collector_blobs(output)
     for raw_json in matches:
-        collector = json.loads(raw_json)
+        try:
+            collector = json.loads(raw_json)
+        except Exception as e:
+            raise Exception(f'Error loading json: {e}\nCollector Json Output:\n{raw_json}')
         replay_check_run(collector, aggregator, datadog_agent)
 
     return len(matches)