Flaky test failure in Package Tests (Lowest Dependencies)

[zhumin8]

During GitHub Actions CI execution for Package Tests (Lowest Dependencies), the Gax unit test suite intermittently aborts with a core dump due to an absl::Mutex deadlock cycle detected by the underlying gRPC C-extension.

Failed Job Details
Step: Run Package Test Suites -> Processing Gax
Sample Failed Run: https://github.com/googleapis/google-cloud-php/actions/runs/35243107402/job/105276300501
Associated PR: #9700 (Rerun passed)
Also observed in #9701.

Error Logs

--- Processing Gax ---
Use local package Auth as google/auth:1.100 in Gax
Use local package CommonProtos as google/common-protos:4.100 in Gax
Installing composer in Gax (with --prefer-lowest)
Running Gax Unit Tests
...
[mutex.cc : 1425] RAW: Potential Mutex deadlock:  
[mutex.cc : 1435] RAW: Acquiring absl::Mutex 0x55f634b93d90 while holding  0x55f634b6e390 0x55f634b7df70 0x55f6349a0330; a cycle in the historical lock ordering graph has been observed
[mutex.cc : 1439] RAW: Cycle: 
[mutex.cc : 1453] RAW: mutex@0x55f634b93d90 stack: 
[mutex.cc : 1453] RAW: mutex@0x55f634b7df70 stack: 
[mutex.cc : 1461] RAW: dying due to potential deadlock
environment: line 1: 62211 Aborted                 (core dumped) "${DIR}/vendor/bin/phpunit" -c "${DIR}/phpunit.xml.dist"

...
--------- Failed tests --------------
Gax: failed
-------------------------------------
##[error]Process completed with exit code 1.