fix(workflow): preserve event.output for cached LlmAgent results

[ValentinCordonnier]

Fixes #5553

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Summary

Runner._consume_event_queue was nulling event.output before persisting any non-partial event whose node_info.message_as_output was True. The intent looked like a denormalization step ("the LLM text already lives in event.content, no need to store it twice"), but it broke resume rehydration. On the live path, process_llm_agent_output populates event.output with the validated dict (or raw text, when no output_schema); after the strip, the persisted event lost that value. On resume, _reconstruct_node_state walked the persisted events, found event.output is None, and fell through to child.output = event.content — handing the orchestrator a raw genai.types.Content instead of the dict. Anything subscripting that result (lab[\"findings\"], Schema.model_validate(out), …) crashed with 'Content' object is not subscriptable only after a HITL pause, even though the same code worked on the initial pass.

The diagnosis in #5553 by @samarth1224 already pinned the two functions involved; this PR is the minimal change.

Changes

• src/google/adk/runners.py: removed the four-line strip block in _consume_event_queue that copied the event and assigned event.output = None for non-partial message_as_output events. With the strip gone, event.output survives the persist / rehydrate roundtrip and _reconstruct_node_state's primary branch (if event.output is not None: child.output = event.output) returns the same Python object type the live path produced.
• tests/unittests/workflow/test_workflow_hitl.py: added a regression test that pauses a rerun_on_resume=True orchestrator on a RequestInput, resumes, and subscripts the cached LlmAgent output. Fails on v2 HEAD with the reported TypeError; passes after this change.
• tests/unittests/workflow/test_workflow_dynamic_nodes.py: updated test_workflow_resume_does_not_rerun_completed_llm_agent. Its previous assertion agent_event.output is None (commented "Verify that runners.py cleared the output") was asserting the buggy behavior as if intentional. Replaced with a positive check that the persisted output equals the agent's emitted text. The test's actual semantic guarantee — the LLM is not re-fired on resume — still holds via the existing agent_runs_again == [] assertion.

Motivation

Any @node(rerun_on_resume=True) orchestrator that subscripts an LlmAgent result via ctx.run_node and survives a RequestInput pause is silently broken on v2. Graph-edge propagation of LlmAgent outputs across resume has the same failure mode (the rehydrated child.output is a Content instead of the schema dict), so this isn't only a dynamic-orchestrator problem. The fix unblocks the entire HITL-with-structured-output pattern, which is a core v2 use case.

What does not change

• Live-path semantics. _node_runner._enqueue_event runs before this strip in the queue and was already reading event.output correctly; non-resume runs are unaffected.
• Caching policy. rerun_on_resume, dedup of completed nodes, and the _dynamic_node_scheduler state machine are all unchanged. The fix only changes what value gets returned from the cache, not whether the cache is hit.
• The event.content fallback in _reconstruct_node_state is preserved (it now handles only legacy persisted events).
• No public API surface changes. Event.output: Any | None is unchanged; no contract said it had to be None for LLM events.

Downsides

• Slightly larger persisted events for LlmAgent final responses. The validated dict is stored alongside the original text in content.parts. For output_schema agents that's roughly the JSON serialization of the dict; for plain-text agents it's roughly the response text duplicated. The previous saving was illusory — rehydration was returning a different object type than the live path, which is the bug we're fixing — but storage-cost-sensitive deployments will see the bytes-per-event grow.
• Plugin behavior change. Plugins that read event.output for LLM events used to see None; they will now see the dict / string. Any plugin that branched on event.output is None to detect "this is the final LLM event" needs to switch to event.node_info.message_as_output (unchanged, set by the live path). I couldn't find any in-tree plugin doing this, but third-party plugins exist.
• Old persisted sessions stay broken. Sessions persisted by pre-fix runners still have output=None for these events; resuming them post-fix continues to hit the event.content fallback. That's not a regression — those sessions were broken before — but cross-version resume is not magically fixed. The fallback branch could be removed in a follow-up once legacy sessions age out.
• One existing test asserted the buggy behavior and had to be updated (see the third bullet under Changes). Reviewers should sanity-check that the replacement assertion still expresses the test's true semantic intent.

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