Fix managed multisite PHPUnit composition

[chubes4]

Summary

• preinstall managed MySQL multisite with wp-phpunit before enabling installed-network constants
• explicitly materialize prepared extra plugins into Playground VFS before setup and workloads
• preserve fresh structured diagnostics when multisite preinstall fails

Root Cause

The managed runner defined MULTISITE before wp-phpunit's installer could create and seed network tables. WordPress therefore queried blogs and sitemeta during bootstrap before those tables existed. Separately, prepared extra plugins were mounted but omitted from explicit staged-input materialization, so sibling dependencies were not guaranteed to exist inside Playground.

Implementation

• Generate one shared managed wp-tests configuration for preinstall and normal execution.
• Run wp-phpunit's existing install.php with run_ms_tests in a separate preinstall invocation that excludes current-network constants.
• Keep the normal managed PHPUnit lifecycle unchanged after the network exists.
• Initialize and persist preinstall throwable/fatal diagnostics through the normal structured result path.
• Include generic prepared extra-plugin mounts in materializeStagedInputs() without consumer-specific knowledge.

Verification

• npm run build
• npx tsx tests/phpunit-project-autoload.test.ts
• npm run test:recipe-runtime-setup-staged-materialization
• npm run test:playground-phpunit-readonly-cache-integration
• npm run check passed production-boundary enforcement, build, and generic primitives before stopping at the existing main-branch command-registry-smoke failure: wordpress.collect-workload-result outputShape should mention outputSchema id
• npm run test:disposable-mysql-mysqli-e2e skipped because Docker is unavailable locally; CI provides the decisive MySQL coverage
• git diff --check

Fixes #2056