Execute bounded PHP commands through clean Playground processes

[chubes4]

Problem

Bounded runtime entries with processIdentity require isolated PHP state while reusing one Playground server, its mounted filesystem, extensions, and runtime services.

The current HTTP request-worker approach does not provide that isolation: the request worker may already have loaded the managed WordPress installation. A project-owned PHPUnit bootstrap that loads its own WordPress tree then fails on inherited symbols such as add_filter(). Generic proc_open() spawning remains inside the same PHP-WASM environment and does not solve the collision.

Project bootstrap mode also currently prepends the managed /wordpress/wp-load.php, which violates project ownership, while PHPUnit configurations without a bootstrap are rejected even though native PHPUnit accepts them.

Upstream dependency

WordPress Playground issue: WordPress/wordpress-playground#4145
WordPress Playground PR: WordPress/wordpress-playground#4146

The Playground PR exposes runInFreshProcess() with preserved mounts/configuration and collision-free concurrent code execution.

Required changes

• Route bounded code executions with a process identity through playground.runInFreshProcess().
• Remove the HTTP request-worker endpoint and payload-file transport.
• Preserve per-entry runtime environment injection, including bootstrap=none.
• Let project bootstrap mode own WordPress loading instead of prepending managed wp-load.php.
• Permit project-mode PHPUnit configurations that declare no bootstrap.
• Fail clearly when the selected Playground backend lacks clean-process support.

Acceptance criteria

• A project bootstrap may define add_filter() without colliding with managed WordPress symbols.
• Concurrent bounded entries retain isolated PHP state and their own service environment.
• Existing non-bounded Playground commands retain current behavior.
• Focused runner/bootstrap tests pass.
• Downstream TeamCity-equivalent validation passes all 680 PHPUnit suites with zero failures, timeouts, or cancellations.