LangGraph graphs with checkpointed/multi-turn state fail the spawn-safety probe

**Files:** `src/conductor/ai/agents/runtime/runtime.py` (`_register_passthrough_worker`), `src/conductor/ai/agents/runtime/_worker_entries.py` (`probe_spawn_safety`)

**Symptom:** `python examples/agents/langgraph/03_memory.py` fails in <1s, client-side, before any server call:
```
SpawnSafetyError: worker 'memory_agent' is not spawn-safe
AttributeError("Can't get local object 'CompiledStateGraph.attach_node.._get_updates'")
Define the callable at module level (importable by qualified name).
```

**Cause:** The passthrough worker pickles the compiled graph. LangGraph's `CompiledStateGraph` contains a local closure, so it can never be picklable. The error's advice is unfollowable — the object belongs to LangGraph, not to user code.

**Fix:** Either avoid pickling the compiled graph in the framework passthrough path, or detect this graph shape and fail with a message naming a supported alternative.

**Verify:** `python examples/agents/langgraph/03_memory.py` runs. Same for `13_multi_turn.py` and `27_persistent_memory.py`.

**Scope:** 3 of 46 langgraph examples. All three use checkpointed or multi-turn state; the other 43 pass.

Start with _register_passthrough_worker in src/conductor/ai/agents/runtime/runtime.py and probe_spawn_safety in src/conductor/ai/agents/runtime/_worker_entries.py, then reproduce the failure with examples/agents/langgraph/03_memory.py. Compare the checkpointed and multi-turn examples with a passing LangGraph example, and verify that 03_memory.py, 13_multi_turn.py, and 27_persistent_memory.py run successfully without the spawn-safety failure.

Rédigé par le modèle d'indexation à partir du texte de l'issue.