[Sampler.AWS] Fix flaky tests

[martincostello]

Fixes #1219

Changes

• Refactor to avoid test flakiness on disposal.
• Enable test that now works on .NET Framework.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.85%. Comparing base (a545405) to head (a46b169).
⚠️ Report is 11 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4277   +/-   ##
=======================================
  Coverage   73.85%   73.85%           
=======================================
  Files         459      459           
  Lines       18327    18327           
=======================================
  Hits        13535    13535           
  Misses       4792     4792           

Flag	Coverage Δ
unittests-Sampler.AWS	94.27% <ø> (ø)

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[Copilot]

Pull request overview

This PR addresses flakiness in OpenTelemetry.Sampler.AWS tests (issue #1219) by making the mock HTTP server interactions and polling-based assertions more resilient, and by enabling a previously skipped .NET Framework test.

Changes:

• Refactors test server handler wiring (method-group) and response writing/closing behavior to reduce disposal-related flakiness.
• Converts TestSamplerUpdateAndSample to async Task and replaces fixed Thread.Sleep waits with async waiting/polling.
• Removes the NETFRAMEWORK skip so the sampler update test runs on .NET Framework again.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
test/OpenTelemetry.Sampler.AWS.Tests/TestAWSXRaySamplerClient.cs	Uses method-group delegate for the test HTTP server handler.
test/OpenTelemetry.Sampler.AWS.Tests/TestAWSXRayRemoteSampler.cs	Re-enables .NET Framework test; replaces sleeps with async delay and adds polling to reduce timing flakiness.
test/OpenTelemetry.Sampler.AWS.Tests/MockServerRequestHandler.cs	Adds try/finally to ensure responses are closed; uses leaveOpen writer to avoid stream disposal issues.

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