fix(runtime): resolve transfer_task caller from pinned session agent

[yangtuooc]

What

Fixes transfer_task failing inside a run_background_agent sub-session. See issue #3886.

Problem

A coordinator (root) fans out work via run_background_agent to a composite agent (pipeline) that has its own sub_agents (e.g. director). When pipeline calls transfer_task(agent="director"), it fails with:

Agent root cannot transfer task to director: target agent not in sub-agents list. Available agent IDs are: pipeline

The error names root as the caller even though the call originates from pipeline, whose sub-agents do include director.

Root cause

handleTaskTransfer resolves the calling agent via r.CurrentAgent(), which returns the runtime's shared current-agent field. Background sub-sessions are created with PinAgent: true + WithAgentName(cfg.AgentName) (RunAgent → runCollecting), which pins the session to pipeline but deliberately does not mutate currentAgent (it stays root). So validation runs against root.SubAgents() = [pipeline] and rejects director.

agentRouter.ResolveSession already handles pinned sessions ("when sess pins a specific agent (e.g. background agent tasks), that agent is returned directly"), but handleTaskTransfer wasn't using it.

Fix

Resolve the caller via resolveSessionAgent(sess) instead of CurrentAgent(). This returns the pinned agent when the session pins one, and falls back to CurrentAgent() otherwise — so ordinary transfer_task behavior is unchanged.

Tests

Adds TestHandleTaskTransfer_UsesPinnedSessionAgent: a session pinned to pipeline (as run_background_agent does) transfers to director while the runtime's current-agent is root.

• Without fix: fails with the exact error above
• With fix: passes

go test ./pkg/runtime/ passes.

Closes #3886