Fix #930: mock substitution hangs when TestNodeConfigs is absent (#1086)

Make TestNodeConfigs parsing conditional on its presence in the JSON
configuration. Previously, an empty TestNodeConfigs object was always
created, causing mock nodes to be registered even when no test
configuration was intended, which led to infinite loops during tree
execution.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: facontidavide