fix(runner): a source that was asked and handed nothing says so

NextAction returning ErrNoAction left the step with no skip reason, so a
run whose every model call failed on transport, a non-2xx, an empty
choices array or an echo mismatch printed no violations and exited 0.
Only llm-calls.jsonl knew it had never touched the app.

The reason now travels the path the other five already take, so it
reaches the trace, the summary, and the campaign's dispatched-action
exclusion. A held step never asks and keeps carrying nothing.

internal/runner/testdata/violation-summary.txt

@@ -2,3 +2,4 @@
 run complete: 4 steps
 1 violation record(s):
   step 1: [balanceNonNegative]
+4 action(s) never reached the app: no_action_produced 4