Execute bounded PHP commands in clean Playground processes

[chubes4]

Fixes #1937.

Depends on WordPress/wordpress-playground#4146.

What changed

• Routes bounded code executions with processIdentity through Playground's runInFreshProcess().
• Removes the HTTP request-worker endpoint and payload-file transport.
• Preserves runtime and secret environment injection for bootstrap=none.
• Lets project bootstrap mode own WordPress loading instead of prepending managed /wordpress/wp-load.php.
• Permits project-mode PHPUnit configurations that do not declare a bootstrap.
• Fails clearly when a selected Playground backend does not expose clean-process execution.
• Decodes the bootstrap diagnostic wrapper in project-autoload assertions added on current main.

Verification

• npm run build
• npx tsx tests/playground-cli-runner-bootstrap-ini.test.ts
• npx tsx tests/phpunit-project-autoload.test.ts
• Combined downstream acceptance using the Playground #4146 package: 680/680 TeamCity-equivalent PHPUnit suites passed at concurrency 64, with zero failures, timeouts, or cancellations.

Merge dependency

The code intentionally fails closed for bounded process identities until the selected Playground backend provides runInFreshProcess(). Merge/release coordination should therefore follow WordPress/wordpress-playground#4146.

[chubes4]

Full-stack proof update for the clean-process PHPUnit path:

• WP Codebox head: 3fad3386
• Reproducible combined Playground proof ref: https://github.com/chubes4/wordpress-playground/tree/proof/bet12-clean-worker-abi
• Upstream dependencies: Fix external PHP extension ABI exports WordPress/wordpress-playground#4108 and [CLI] Run PHP in clean worker processes WordPress/wordpress-playground#4146
• The current contracts CI failure is the expected dependency boundary: published Playground 3.1.46 does not contain runPHPInNewProcess, so bounded workers fail with Cannot read properties of undefined (reading apply). The combined proof ref passes the PHP 8.4 server-first TCP regression.

Using that combined Playground build plus this PR on Homeboy Lab, real focused WPCOM PHPUnit execution passed under the new non-zero test-summary gate:

• MediaCreateAbilityIntegrationTest: 12 tests, 50 assertions
• JetpackClientActionUploadAudioTest: 6 tests, 13 assertions
• JetpackClientActionUploadImageTest: 7 tests

The unfiltered wpcom-mcp-abilities suite also executed all 443 tests; it reported 11 unrelated locale-environment errors rather than a false process-level success. This PR now rejects zero-exit PHPUnit runs that do not contain a non-zero OK (...) summary.