test(core): harden timing-sensitive process and project-copy tests - #48205

Issue for this PR

Closes #37321

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Fixes the two timing flakes from the CI run in #37321. Test-only changes, no product code touched.

1. packages/core/test/process/process.test.ts: waitForFile returned the first successful read. The child writes the readiness file with writeFileSync, which creates the file before its contents land, so the helper could observe it right after creation and return "" (Expected: /^\d+$/ Received: ""). It now only returns once the file is non-empty, sleeping 10ms between attempts either way. All four call sites expect non-empty contents (PID regex twice, "settled" twice), so no legitimate empty read exists.

2. packages/core/test/project-copy.test.ts: refresh ignores stale git worktree registrations runs two git worktree add calls plus a recursive remove on top of setup()'s git init, which blows the default live-test timeout on loaded Windows runners. Bumped to 15s with a comment saying why.

Prior attempt #37377 did the same two things but was closed unmerged without review; this re-applies them on current dev, where the flaky helper is still byte-identical to the version in the issue.

How did you verify your code works?

• bun test test/process/process.test.ts in packages/core: 24 pass, repeated 3x, 0 fail.
• bun test test/project-copy.test.ts in packages/core: 13 pass, repeated 3x, 0 fail.
• bun typecheck in packages/core: clean.

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR