## Why
Test fixture setup can take more than five seconds on loaded CI workers, and
failures returned from setup can be obscured when downstream mocks verify their
expectations during teardown.
## What changed
- Add a shared `expect_startup` helper with a 30-second timeout that panics on
either timeout or setup failure before mock teardown runs.
- Use the helper for remote-environment and unified-exec fixture setup, and
allow the same startup window for the initial exec-server requests.
## Testing
Add paused-time tests covering successful startup after five seconds, timeout
reporting, and propagation of setup errors in the presence of unmet mocks.
GitOrigin-RevId: 39db69bde2983acd023a1911a6fad106674e169a
8aea62b2d8