Add --endpoint flag to agentcore invoke to target specific runtime endpoints

[jariy17]

Summary

Users can create and deploy runtime endpoints (named version aliases) but cannot invoke them individually from the CLI. agentcore invoke only supports --target for deployment targets, with no way to address a specific endpoint.

Current behavior

agentcore invoke --help | grep endpoint
# (nothing)

Expected behavior

agentcore invoke --endpoint prod "Hello"

Should resolve the endpoint ARN from deployed-state.json and route the invocation to that specific endpoint.

Context

Runtime endpoints let users pin traffic to specific runtime versions (e.g., prod → v3, staging → v5). Without --endpoint on invoke, users can create endpoints but not test them from the CLI.