fix(test): Use async sleep in stresstest

[jan-auer]

std::thread::sleep blocks the tokio runtime thread. Use tokio::time::sleep to yield back to the executor while waiting for the server to start.

Since this is in tests, this is mostly a cosmetic change. I'm changing this for best practices.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.47%. Comparing base (4b0fe8f) to head (2570dbc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #509   +/-   ##
=======================================
  Coverage   87.47%   87.47%           
=======================================
  Files          86       86           
  Lines       13912    13912           
=======================================
  Hits        12169    12169           
  Misses       1743     1743           

Components	Coverage Δ
Rust Backend	92.26% <ø> (ø)
Rust Client	80.06% <ø> (ø)
Python Client	88.51% <ø> (ø)

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.