ci: wait for Batfish readiness before running lab tests

[dhalperi]

Replace fixed startup sleeps with an endpoint readiness probe. Fail
early when the Batfish process or container exits before becoming ready.

---

Prompt:

Build keeps failing on batfish not launched. Can we make the launchng process more robust? https://github.com/batfish/lab-validation/actions/runs/36918043504/job/110557367647#step:9:945

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.59%. Comparing base (9f01e12) to head (27787fe).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #241   +/-   ##
=======================================
  Coverage   83.59%   83.59%           
=======================================
  Files          96       96           
  Lines        4706     4706           
=======================================
  Hits         3934     3934           
  Misses        772      772           

Flag	Coverage Δ
unittests	83.59% <ø> (ø)

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