feat(appkit): auto-start app under test for agent evals (stack 6/6)

[MarioCadenas]

Stack 6/6 · targets pr/agent-evals-5-docs (review after #481).

Playwright-style auto-start for the app under test, so appkit agent eval no longer requires a manually-started server.

• Root evals.config.ts (project root, via defineEvalConfig) carries run-wide settings — baseUrl and webServer { command, url?, timeoutMs?, reuseExisting? } — plus defaults for maxConcurrency/timeoutMs. loadRootEvalConfig/findRootEvalConfig exposed from @databricks/appkit/beta. The per-agent evals.config.ts keeps its narrow role (additive — nothing removed).
• CLI lifecycle: resolves baseUrl/concurrency/timeout as flag > root config > default; before running, reuses a server already answering at url (default) or spawns command, polls until any HTTP response or timeout, then SIGTERMs the process group in a finally. A reused server is left running.
• --url default 3000 → 8000 (the port these apps actually serve on).

Design was grilled to resolve scope (root config, spawn in CLI not the library, readiness = any HTTP response, reuse-existing default true). Verified end-to-end against dev-playground: cold start boots + tears down; warm start reuses and leaves the dev server up.

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📦 Bundle size report

Compared against bundle-size-baseline.json (main).

@databricks/appkit

npm tarball (packed): 1.1 MB (+15 KB) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	1.1 MB (+13 KB)	411 KB (+4.5 KB)
Type declarations	426 KB (+8.1 KB)	153 KB (+3.0 KB)
Source maps	2.3 MB (+25 KB)	770 KB (+8.5 KB)
Other	11 KB	3.7 KB
Total	3.8 MB (+46 KB)	1.3 MB (+16 KB)

Per-entry composition (own code — deps external (as shipped))

Entry	Initial (gz)	Lazy (gz)	Total (gz)	node_modules (min)	Own code (min)
.	95 KB	2.5 KB	98 KB	external	313 KB
./beta	92 KB (+1.5 KB)	457 B	92 KB (+1.5 KB)	external	278 KB (+4.1 KB)
./testing	17 KB	0 B	17 KB	external	52 KB
./tsdown	520 B	0 B	520 B	external	813 B
./type-generator	23 KB	0 B	23 KB	external	65 KB

Chunks:

Entry	Chunk	Load	Size (gz)
.	index.js	initial	91 KB
.	utils.js	initial	4.0 KB
.	remote-tunnel-manager.js	lazy	2.5 KB
./beta	beta.js	initial	76 KB
./beta	stream-manager.js	initial	5.8 KB
./beta	wide-event-emitter.js	initial	3.2 KB
./beta	databricks.js	initial	3.2 KB
./beta	configuration.js	initial	2.3 KB
./beta	service-context.js	initial	1.3 KB
./beta	client.js	initial	434 B
./beta	client-options.js	initial	220 B
./beta	supervisor-api.js	lazy	192 B
./beta	databricks.js	lazy	142 B
./beta	index.js	lazy	123 B
./testing	index.js	initial	17 KB
./tsdown	index.js	initial	520 B
./type-generator	index.js	initial	23 KB

@databricks/appkit-ui

npm tarball (packed): 350 KB (-4 B) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	395 KB	132 KB
Type declarations	229 KB	84 KB (-1 B)
Source maps	766 KB	253 KB
CSS	16 KB	3.2 KB
Total	1.4 MB	473 KB (-1 B)

Per-entry composition (consumer bundle — deps bundled, peerDeps external)

Entry	Initial (gz)	Lazy (gz)	Total (gz)	node_modules (min)	Own code (min)
./js	5.3 KB	49 KB	55 KB	208 KB	14 KB
./js/beta	20 B	0 B	20 B	0 B	0 B
./react	432 KB	49 KB	481 KB	1.3 MB	177 KB
./react/beta	1.0 KB	0 B	1.0 KB	0 B	1.9 KB

Chunks:

Entry	Chunk	Load	Size (gz)
./js	index.js	initial	5.2 KB
./js	chunk	initial	120 B
./js	apache-arrow	lazy	49 KB
./js/beta	beta.js	initial	20 B
./react	index.js	initial	430 KB
./react	tslib	initial	2.1 KB
./react	apache-arrow	lazy	49 KB
./react/beta	beta.js	initial	1.0 KB

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📦 Try this PR's app template

Scaffolds a new app from this PR's SDK build. Run it in any folder (requires the GitHub CLI — gh auth login — and the Databricks CLI):

gh run download 34379662677 -R databricks/appkit -n appkit-template-0.73.0-pr.58372b2-pr-agent-evals-6-webserver-482 -D appkit-pr-482 \
  && unzip -o "appkit-pr-482/appkit-template-0.73.0-pr.58372b2-pr-agent-evals-6-webserver-482.zip" -d "appkit-pr-482" \
  && databricks apps init --template "appkit-pr-482"

The template pins @databricks/appkit and @databricks/appkit-ui to tarballs built from this branch, so the scaffolded app runs against this PR's code.