fix: skip delegation nudge in subagent sessions

When a user message contains an @agent part, createUserMessage injects a
synthetic instruction telling the model to call the task tool. This nudge
is intended for root sessions, but it also fires inside subagent sessions,
causing recursive nesting bias: subagents that receive @agent mentions in
their context interpret them as new delegation requests rather than
informational references.

Fix: fetch the current session in createUserMessage and skip the synthetic
text injection when session.parentID is set (i.e. the session is a child
of another session). The agentPart itself is still returned so the mention
is preserved in the message history.

Fixes #17202

Author: vicary