Two error handling gaps in the gateway kanban dispatcher:
1. float() on dispatch_interval_seconds crashes with ValueError if the
config value is a non-numeric string. Wrap in try/except and fall
back to the default 60-second interval with a warning log.
2. splitlines()[0] on payload_summary and task.result raises IndexError
when the string is whitespace-only (truthy but strip() produces empty
string, splitlines() returns []). Guard with a check on the lines
list before indexing.
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