test(core): flaky AppProcess interruption tests (readiness race) #51951

Description

packages/core/test/process/process.test.ts is flaky on a loaded machine. The two AppProcess interruption tests ("fiber interruption cleans up the scoped child process after readiness" and the timeout variant) sometimes fail with:

expect(yield* waitForFile(ready)).toMatch(/^\d+$/)
Received: ""

The child script writes the ready file before registering its SIGTERM handler, and writeFileSync creates the file empty before writing the pid. So the test can read an empty file, or send SIGTERM before the handler exists (the child then dies without writing settled).

OpenCode version

dev

Steps to reproduce

1. cd packages/core
2. Run bun test test/process/process.test.ts 16 times in parallel, in a loop (I used ~60 rounds).
3. A few runs fail as above; running the file alone rarely shows it.

Operating System

macOS (arm64)