Add agentSessionSubagentEnabled setting under experimental.adk that
routes subagent invocations through LocalSessionInvocation and
RemoteSessionInvocation instead of legacy executors. When disabled
(default), all behavior is unchanged.
- Add agentSessionSubagentEnabled to ADKSettings, Config, and settings schema
- Thread onAgentEvent callback through AgentTool → DelegateInvocation
- Route local/remote agents to session invocations when flag is enabled
- Browser agent always uses BrowserAgentInvocation regardless of flag
- Copy local protocol files needed for the session invocation path