wasp-app-runner: Run client and server builds in parallel before starting apps

[Copilot]

Description

The client build completes much faster than the server build, causing the Vite URLs to be scrolled away by server build logs. This changes the execution order from parallel([series([clientBuild, clientStart]), series([serverBuild, serverStart])]) to series([parallel([clientBuild, serverBuild]), parallel([clientStart, serverStart])]).

Changes:

• Refactored client.ts and server.ts to expose separate build* and start* functions
• Updated build/index.ts to use Promise.all() for parallel builds, then start both apps
• Added proper error handling with safe property access for promise rejections
• Added infinite promise to keep main process alive while child processes run

Both applications now start at roughly the same time, preventing the Vite URLs from being buried in server build output.

Type of change

• 🐞 Bug fix

Checklist

• I tested my change in a Wasp app to verify that it works as intended.

• 🧪 Tests and apps:

  • I added unit tests for my change.
  • (if you fixed a bug) I added a regression test for the bug I fixed.

• 📜 Documentation:

  • (if you added/updated a feature) I added/updated the documentation in web/docs/.

• 🆕 Changelog:

  • I updated waspc/ChangeLog.md with a user-friendly description of the change.
  • (if you did a breaking change) I added a step to the current migration guide in web/docs/migration-guides/.
  • I bumped the version in waspc/waspc.cabal to reflect the changes I introduced.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>wasp-app-runner build: Wait until both builds are done before running them</issue_title>
<issue_description>Right now wasp-app-runner does (pseudo code):

parallel([
  series([ clientBuild, clientStart ]),
  series([ serverBuild, serverStart ])
])

This leads to the client usually being ready much sooner than the server, and the Vite message with the URLs being scrolled out by the server build logs.

I think it'd be much better to do:

series([
  parallel([ clientBuild, serverBuild ]),
  parallel([ clientStart, serverStart ])
])

So both sides start more or less at the same time.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes wasp-app-runner build: Wait until both builds are done before running them #3498

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