test(instrumentation-http): make timing dependent test more robust

[cjihrig]

Which problem is this PR solving?

The test should have 2 ended span when client prematurely close relies on specific timing of asynchronous events. This leads to it being flaky in the CI.

Fixes # https://github.com/open-telemetry/opentelemetry-js/actions/runs/19544684677/job/55960292192

Short description of the changes

This commit updates the test to use a polling mechanism instead of relying on timers of specific durations.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Reproduced the issue locally and then updated the test.

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.21%. Comparing base (3110287) to head (978b9fd).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6144   +/-   ##
=======================================
  Coverage   95.21%   95.21%           
=======================================
  Files         316      316           
  Lines        9389     9389           
  Branches     2167     2167           
=======================================
  Hits         8940     8940           
  Misses        449      449           

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[otelbot]

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