Playwright: pre-build test apps before webServer startup

[nicoburns]

Summary

Playwright previously started ~20 dx run/cargo run webServers concurrently, each doing a full build at startup — ~20 cargo invocations racing over CPU and the target-dir lock, a major source of CI slowness and flaky server-startup failures.

This extends the existing _DX_BUILT one-time init block in playwright.config.js: after building dx, it now pre-builds every active test app sequentially (cargo parallelizes internally), so webServer startup is launch-only. The build command is derived from each server's run command:

preBuildCommand(s):
  command includes "--hot-patch"  -> null (skip: fat build isn't shared with plain `dx build`)
  "cargo run ..."                 -> "cargo build ..."          // liveview
  "dx run <flags>"                -> "dx build <flags>" minus serve-only `--addr`/`--port`

Keeping the remaining flags (--web, --release, --ssg, --bin, --wasm-split, --profile, --force-sequential) identical means the subsequent dx run hits the same cargo fingerprint and reuses all artifacts. Non-hot-patch dx run/dx serve builds use BuildMode::Base, same as dx build.

webServer timeout drops from 50 min to 10 min for pre-built servers; hot-patch servers (which still do their full fat build at startup) keep 50 min.

Local workflow is unchanged: activeServers already filters to the specs passed on the CLI, so a single-spec run only pre-builds that one app.

Verification

On Linux, ran npx playwright test web.spec.js web-routing.spec.js --reporter=line from a clean target state:

• Pre-build phase compiled both apps sequentially (Pre-building web: ... dx build --force-sequential --web, then web-routing).
• webServer startup did no crate compilation — logs show only Bundling app / Running WASM bindgen / Build completed successfully in 2.08s, launching app!.
• All 19 tests passed (7.0s).

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/1ad909911b2445319691f40122f4f2e9
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/1ad909911b2445319691f40122f4f2e9?variant=devin-insiders
Requested by: @nicoburns

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