Host Everruns serve apps on Amazon Bedrock AgentCore Runtime

The new everruns-serve-agentcore crate hosts a serve app on Amazon Bedrock
AgentCore Runtime. It answers GET /ping and POST /invocations, which take
an AG-UI RunAgentInput or {"prompt": ...} and stream AG-UI events, plus a
/ws WebSocket transport, with serve’s /v1 API alongside.
The session log and agent workspace live on AgentCore session storage. Bedrock
models use the runtime’s execution role when AWS_REGION is set, and parked
approvals and ask_user questions survive a restart.
See Serve on AgentCore. PRs #3990, #4012.