Fix ADK single-turn agent-as-tool hanging on Conductor

[ling-senpeng13]

Problem

An ADK coordinator whose sub-agents declare mode="single_turn" hangs on Conductor — status stays RUNNING, no error, no timeout, no answer. The identical agent runs correctly under adk web / adk run.

Cause

ADK creates the duplicate itself. Declaring a sub-agent mode="single_turn" auto-inserts a private tool wrapper into the parent's tools while still listing that agent in sub_agents. Serializing the reported coordinator shows both fields carrying the same two children:

tools:      [(_SingleTurnAgentTool, weather_specialist), (_SingleTurnAgentTool, time_specialist)]
sub_agents: [(LlmAgent,             weather_specialist), (LlmAgent,             time_specialist)]

ADK reconciles this per request — _get_transfer_targets() drops single-turn sub-agents when it assembles the model call. The server compiles from a serialized snapshot, where no such step exists, so both survive. It sees a coordinator with tools and sub-agents and offers the model two routes per child with incompatible semantics: the bare sub-agent name, forked as a SIMPLE task no worker is registered for (SCHEDULED forever, JOIN never completes), or a transfer signal that ends the loop and hands off permanently to one specialist. Hence either a hang or a partial answer.

Compounding it, the serializer didn't recognise the wrapper at all: agent-as-tool detection gated on _is_agent_tool, an OpenAI-SDK attribute the ADK wrapper lacks, so it fell through to generic pydantic walking and emitted the private class name as its _type.

Fix

Two changes in serializer.py, both keyed on one isinstance check against ADK's exported AgentTool base class — which covers the private subclasses for single_turn and task without naming them, and won't break on an ADK rename:

• Emit ADK agent-tools under the public AgentTool shape.
• Drop sub-agents already reachable as an agent-tool from sub_agents, keyed on object identity since ADK wraps the very same instance.

Adds an e2e regression test, guarded by importorskip on google-adk.