#977 Undefined template path renders as an empty string without any warning

EARS: If a {{path}} placeholder in a task argument resolves to no value, then the Highway engine shall either fail the task with an error naming the path or record a warning event visible through the API. EVIDENCE (#961): workflows/parallel_demo_fixed draft used {{api_response.status_code}} (the http tool result has no such key); the command ran as echo 'API data fetched: HTTP' and the run completed (002ed2d3). A typo silently changes data. resolver.py interpolate_value returns '' for None.