* fix(runtime): stop subgraph stream frames impersonating root frames
The web frontend always requested stream_subgraphs, and since delegated
subagent graphs inherit the parent checkpoint namespace (#4215), their
values snapshots and token chunks ride the parent stream. The worker's
_unpack_stream_item dropped the namespace and published every subgraph
frame under a bare event name, so a subagent's values snapshot replaced
the whole thread view in SDK clients (#4399), its token chunks flooded
the parent message stream, and a subagent's LLM error fallback could be
mistaken for the parent run's.
Publish subgraph frames under namespace-qualified SSE event names
(mode|ns1|ns2, LangGraph Platform style) and keep root-only consumers
(file-tool chunk batcher, subagent event persistence, error-fallback
detection) on root frames only. Drop streamSubgraphs from the frontend
submit paths: subtask progress arrives via root-namespace task_* custom
events, so the flag only exposed the leak.
* test(runtime): add production-shaped subgraph stream regression tests
Address review: the namespace tests validated the publishing helpers
with hand-fed namespaces, while the #4399 regression lived in the
integration between LangGraph's delegation routing and the worker's
stream loop. Add TestWorkerSubgraphStreamIntegration: a real parent
graph delegates through the real SubagentExecutor and streams through
run_agent into a real MemoryStreamBridge, locking both stream_subgraphs
modes -- delegated frames arrive namespaced (never bare), a delegated
error fallback cannot mark the parent run as errored, and without the
flag delegated frames stay out while task_* custom events remain.
