feat(workflow): park parent on WaitForOutput child with no output

[wolo-lab]

Problem

NodeConfig.WaitForOutput existed but was never honored — no production code read it (the M3 finding in the v2 audit). A child dispatched via RunNode that opts into WaitForOutput but finishes without producing output (e.g. a task-mode LlmAgent that delegated to a sub-agent and has not yet yielded its final answer) returned the zero value, falsely completing the parent instead of waiting for it to finish.

Solution

RunNode (the dynamic sub-scheduler path) now honors WaitForOutput. When a child opts in and emits no output (and is not interrupting via HITL), the parent is parked with ErrNodeInterrupted instead of completing, so it re-runs and re-invokes RunNode once the child can produce output.

• Output presence is tracked via a hasOutput flag (nil is a valid output, distinct from "no output").
• waitsForOutput reads the tri-state NodeConfig.WaitForOutput (nil ⇒ engine default false).
• All parking paths share one pause helper.

Relationship to #1037 (WithRaiseOnWait)

This PR is rebased onto #1037 and the two parking mechanisms are unified into a single, consistent path in runNode:

• feat: add WithRaiseOnWait option #1037 — WithRaiseOnWait (per-call flag): parks when the child finishes with an unresolved long-running tool (e.g. a tool awaiting ctx.RequestConfirmation).
• feat(workflow): park parent on WaitForOutput child with no output #986 — WaitForOutput (per-node field): parks when the child finishes with no output.

Both gates run independently and both return through the shared pause helper:

if opts.raiseOnWait && len(pendingLongRunningIDs) > 0 {
    interrupted = true
}
if interrupted { // HITL + raise-on-wait
    return nil, s.pause(name, childPath, runID)
}
if !hasOutput && waitsForOutput(child) { // WaitForOutput
    return nil, s.pause(name, childPath, runID)
}

adk-python parity

adk-python has both, and so do we now:

• NodeConfig.WaitForOutput mirrors adk-python's wait_for_output node field (_base_node.py) — parks unconditionally on missing output.
• WithRaiseOnWait (feat: add WithRaiseOnWait option #1037) mirrors adk-python's per-call run_node(raise_on_wait=True) (agents/context.py) — parks on an unresolved long-running tool.