feat(appkit): supervisor API adapter for agents plugin

[hubertzub-db]

Adds a fourth AgentAdapter that targets the Databricks AI Gateway Responses API, so AppKit apps can host server-side-orchestrated agents (Genie, Knowledge Assistants, UC functions, custom apps, UC connections) without managing tool execution locally.

import { fromSupervisorApi, supervisorTools } from "@databricks/appkit/agents/supervisor-api";

const supervisor = createAgent({
  instructions: "You are an assistant powered by the Databricks Supervisor API.",
  model: await fromSupervisorApi({
    model: "databricks-claude-sonnet-4-5",
    tools: [
      supervisorTools.genieSpace("01ABCDEF12345678", "NYC taxi trip records and zones"),
      supervisorTools.ucFunction("main.default.add", "Adds two integers and returns the sum."),
    ],
  }),
});

What's new

• fromSupervisorApi({...}) — factory that returns an AgentAdapter. Uses the SDK's default credential chain (env, profiles, OAuth, OIDC) just like DatabricksAdapter.fromModelServing.
• supervisorTools.* — concise factories for the five hosted tool types (genieSpace, ucFunction, knowledgeAssistant, app, ucConnection). description is required on every one — it's both validation and the routing hint the LLM uses to choose between tools.
• @databricks/appkit/agents/supervisor-api — new subpath export so consumers pick only the adapter they want; mirrors the existing ./agents/{databricks,vercel-ai,langchain} pattern.

Reference app

Demo supervisor agent in apps/dev-playground/server/index.ts. Tools are commented out by default — uncomment any supervisorTools.* entry to give the model real powers.

Test plan

• Manual tests
• 39 new tests (13 SSE reader + 26 adapter / factory).
• Full appkit vitest suite passes; typecheck clean.