.NET: Investigate and fix persistent durable function integration test failures

[SergeyMenshykh]

Summary

The following durable function integration tests are persistently failing in CI and have been temporarily disabled (skipped):

• ConsoleAppSamplesValidation.LongRunningToolsSampleValidationAsync
• ConsoleAppSamplesValidation.ReliableStreamingSampleValidationAsync

Context

These tests were disabled in #6731 to unblock CI. The root cause of the failures needs to be investigated and resolved so the tests can be re-enabled.

Failed CI run: https://github.com/microsoft/agent-framework/actions/runs/28161194756/job/83401892188

Expected outcome

• Root cause identified and fixed
• Tests re-enabled and passing consistently in CI