Agentic lakehouse: production hardening of the agent gateway

### Task Description

**What needs to be done:**
Externalized session store to unlock multiple replicas, an auth story for the gateway endpoints, refined cost/row/time budgets on tools, and deployment guidance for real clusters.

**Why this task is needed:**
v1 is single-replica with in-memory sessions; production use needs scale-out and access control.

### Task Type
Code improvement/refactoring

### Related Issues
**Parent feature issue:** #19256

Start by reading the parent feature issue #19256 and locating the existing agent gateway implementation described here as v1 with in-memory sessions. Break the work into session storage, endpoint authentication, tool budgets, and cluster deployment guidance; done means each production-hardening area has an agreed implementation and documentation scope.

Written by the indexing model from the issue text.