Add setInitiator()/getInitiator() to agent runner

Prabhavathi Vanipentarequested to merge
issue/ai_agents-3557417:3557417-add-a-way-to-mark-the-executor-on-agent-runner into 1.3.x

Closes #3557417

Introduces setInitiator()/getInitiator() on ConfigAiAgentInterface and AiAgentEntityWrapper, allowing external callers (chatbot modules, custom integrations) to tag an agent run with a string identifying the originating system. The initiator propagates automatically to subagents via executeTool(), is included in all agent events (AgentStartedExecutionEvent, AgentRequestEvent, AgentResponseEventBase, AgentToolBase), and survives toArray()/fromArray() serialization

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