test: remove wall-clock flake from PHPUnit runtime rejection contract

[chubes4]

Problem

tests/phpunit-runtime-rejection.test.ts races terminalization against a fixed 500ms timer and asserts durationMs < 500. Under GitHub Actions scheduling, both push and pull_request contract jobs for #2273 failed at that timer even though the test passes locally and the runtime contract is event-driven.

Failed runs:

• https://github.com/Automattic/wp-codebox/actions/runs/31979523226
• https://github.com/Automattic/wp-codebox/actions/runs/31979525044

Failure:

Error: PHPUnit runtime rejection did not terminalize within 500ms
    at tests/phpunit-runtime-rejection.test.ts:65

Expected

Prove prompt terminalization deterministically from the rejection event without making correctness depend on CI wall-clock scheduling. Keep a bounded deadlock guard, but separate that generous process guard from the semantic assertion.

Acceptance

• Contract remains red if the rejection event does not terminalize the operation.
• Test does not fail merely because a loaded CI runner schedules completion after 500ms.
• Redaction and infrastructure classification assertions remain intact.