[Access] Add architecture, transport, limits, and built-in tools for MCP portals

[kennyj42]

What this PR does

Adds several new sections to the MCP portals page:

• How it works: End-to-end request flow from MCP client through Access auth to upstream server
• Transport: Auto-detection strategy table showing how the portal tries StreamableHTTP, SSE, and URL suffixes
• Built-in portal tools: Documents portal_list_servers, portal_toggle_servers, portal_toggle_single_server, and context optimization tools
• Session lifecycle: 24-hour inactivity timeout, per-session server toggles
• Naming: Disambiguation note explaining Agents Gateway vs MCP Portal naming
• Limits: Resource limits table (20 servers/portal, 5 custom domains, field lengths, etc.)

Why

These are all P2 gaps identified during a comprehensive docs audit:

• Architecture: Internal engineers needed 30+ message threads to explain the routing model (Miguel's question in Core Access Eng, Mar 12)
• Transport: Users do not know what URL format to use or how fallback works
• Built-in tools: Every portal user sees these tools but they are not documented
• Limits: Zero limit documentation existed -- blocks capacity planning
• Naming: 'Agents Gateway' appears in API paths and Terraform but the product is called 'MCP Portal' -- users searching for one do not find the other