test(playground): smoke-test the playground's own server plugins

[IamGalymzhan]

Summary

Third in the stack — base is feat/testing-kit-harness (#540), which is itself based on #530. Review those first; GitHub shows only this PR's 2 files here, and retargets automatically as the parents merge.

The playground's server plugins had no tests at all. 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 entirely unexercised. These five tests cover that side with the harness from #540: real HTTP, faked data plane, no workspace, no credentials, no network.

It also serves as dogfooding that a customer would recognise — the reference app testing its own plugins the way the docs tell them to test theirs.

What's covered

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

Adds a dev-playground vitest project so pnpm test and CI's Unit Tests job pick them up: 4517 → 4522.

Two things writing this turned up

• telemetry-example-plugin really calls fetch("https://example.com") in its external-api span, so a naive test of it needs the internet. The suite stubs non-loopback fetches and passes loopback through — loopback has to work, since that's how the harness reaches its own server. Verified hermetic by running the suite under a socket guard that throws on any non-loopback connect.
• expectStream can't assert on a long-lived stream. It buffers a source to completion and throws on timeout rather than returning partial events. The reconnect stream is five messages three seconds apart, so the SSE test reads one payload off res.body and hangs up instead of costing ~12s.

Notes for the reviewer

• The tests/** exclusion in the new vitest project is load-bearing. Vitest's default **/*.spec.ts glob otherwise collects the Playwright specs, which fail on import with Playwright Test did not expect test.describe() to be called here. Verified by removing it.
• Deliberately smoke-only. Reconnection replay is the real prize — executeStream's ring buffer and Last-Event-ID handling — and deserves its own tests rather than being smuggled into a smoke suite.
• No typecheck script for this app, so the new file is transpile-only. Adding one is out of scope: the existing tsconfig reports 2128 errors from the client's unset --jsx, and a server-scoped one still has 13 pre-existing errors CI has never seen, because pnpm -r typecheck skips this workspace. The most interesting of those 13 is a duplicate-identity clash between appkit/dist/shared/src/workspace-client/types and shared/dist/workspace-client/types — shared is bundled inline into appkit, so the two WorkspaceClient types diverge on Config. Worth its own issue.

Verification

4522 tests pass; typecheck, lint, and format clean. The five new tests pass with zero non-loopback connection attempts under a socket guard.

This pull request and its description were written by Isaac.