backstage-test drives a Models-page read and write through the portal's muster hop; drop the agentPlatform.modelManager mention

[teemow]

Problem

Since giantswarm/backstage#2430 (backstage 2.19.0) the Dev Portal reaches model-manager through the installation's muster as the signed-in person (x_model-manager_* tools) and its backend has no model-manager REST proxy or agentPlatform.modelManager configuration. Two things in agentlab still reflect the old path:

• internal/lab/templates/agent-platform-values.yaml.tmpl explains model-manager's edge route with "the chart's Backstage app-config points the portal backend there (agentPlatform.modelManager.installations.agent-platform.apiBaseUrl)" — no longer true (connectivity chart: stop rendering the dead agentPlatform.modelManager block into the Backstage app-config agent-platform#534 removes the block itself).
• agentlab backstage-test proves the portal's Agent Platform paths as the person (agents, sessions, HITL) but drives no Models-page call: the portal's model-manager reads and writes now take the same POST /api/muster/call hop as agent-manager's, and nothing in the lab asserts it.

Proposed solution

• Reword the template comment: the route stays for model-manager's REST clients (models-test), the portal is not one of them.
• backstage-test: one Models-page read and one write through the portal as the person — x_model-manager_list_backends (the Serving page's first read) and, where the lab runs a host backend with a downloaded model, x_model-manager_load_model / unload_model on it — attributed at muster like the agent-manager calls already are; a viewer's write refused.

Acceptance criteria

• The values template no longer names agentPlatform.modelManager.
• agentlab backstage-test drives the two model-manager calls through the portal's muster hop and reports them like the agent-manager steps.