test(playground): smoke-test the server plugins and showcase the testing kit

[IamGalymzhan]

Stack

• #530 — the testing kit (merged)
  • #540 — createTestApp and the mock client
    • #555 — the kit helpers (env, errors, context, cache, and suite hooks)
      • #542 — the playground tests ← you are here

Each PR targets the one above it, so the diff shown here is only the delta on top of #555. Rebased onto the current #555 tip, so the helpers it adds (useTestApp, the kit-faked workspace client) are available here.

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Summary

The playground's server plugins had no server-side tests. apps/dev-playground/tests/ holds Playwright specs that intercept /api at the browser boundary (page.route + route.fulfill), so the Express server never runs in them — route handlers, execute() interceptors, and executeStream are unexercised.

Two suites cover that side with the harness from #540 — real HTTP, faked data plane, no workspace, no credentials, no network — and dogfood the kit the way the docs tell customers to test their own plugins.

smoke.test.ts — the plugins boot and answer

Test	Proves
all three boot together	the plugin set composes and registers under its manifest names
GET /api/reconnect	the plain JSON route answers
reconnect stream	opens as text/event-stream and emits its first message
POST /api/telemetry-examples/combined	the whole nested-span body runs against the real TelemetryProvider and threads userId through
lakebase-examples unconfigured	degrades to no routes without PGHOST/LAKEBASE_ENDPOINT, and the app still boots

testing-kit.integration.test.ts — a worked example of each recommended helper

One describe per helper, driven against a real plugin where the seam is clean:

Helper	Demonstrated via
getMock (+ responses)	files metadata route → files.getMetadata, asserted on app.client
createMockWorkspaceClient	declared paths answer; undeclared resolve undefined
createApiError	a genuine ApiError, and seeded as a route failure
useTestApp	a fresh reconnect app per test
withEnv	set/restore env around a block
useTestCache / resetTestCache	the real CacheManager — stable & per-user keys, hit-then-miss
createTestPlugin	the genie factory → a configured instance
createTestPluginContext (+ createMockRequest)	cross-plugin tool dispatch, asUser, token-less reject
expectStream (+ createMockResponse)	toEmit / toEmitExactly on a bounded source
useServiceContextMock	the spied data-plane singleton

Where no playground plugin fits hermetically, the demo mirrors the docs: expectStream on a bounded generator (reconnect's stream is long-lived, so smoke.test.ts reads it by hand), and tool dispatch straight through the context (agents needs a live model).

Adds a dev-playground vitest project so pnpm test and CI's Unit Tests job pick both suites up. The tests/** exclusion is load-bearing — without it vitest's default glob collects the Playwright specs, which fail on import.

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This pull request and its description were written by Isaac.