feat(appkit): scan upward from preferred port in development

[MarioCadenas]

Summary

• In NODE_ENV=development, the server plugin now picks the first free TCP port by scanning upward from the preferred port (server({ port }), DATABRICKS_APP_PORT, or default 8000) using get-port's portNumbers(from, to) helper, bounded to 100 attempts.
• Avoids EADDRINUSE failures when running multiple AppKit dev servers locally (e.g. two playgrounds at once).
• Production / non-development behavior is unchanged: the plugin binds the configured / env / default port and lets listen errors surface as before.
• Startup log already prints the bound URL via the existing Server running on http://… line; no extra fallback log is emitted.

Implementation

• New private ServerPlugin.resolveListenPort() returns:
  • In development: await getPort({ host, port: portNumbers(requested, requested + 99) }).
  • Otherwise: the requested port (no scan).
• start() awaits the resolved port before app.listen(...).
• logStartupInfo() reads the resolved port so logs match the actual socket.
• Adds get-port@7.2.0 to @databricks/appkit dependencies.

Test plan

• pnpm --filter @databricks/appkit typecheck
• pnpm exec vitest run packages/appkit/src/plugins/server/tests/server.test.ts packages/appkit/src/plugins/server/tests/server.integration.test.ts (37 tests pass; new dev/prod cases cover the resolution branches via a partial get-port mock)
• pnpm exec biome check --write on touched files
• Manual: start two dev playgrounds back-to-back; confirm second one binds 8001 (or next free port) and the URL line reflects it