Automated fix by @leftibot.

What changed

Fix #666: TSAN builds don't actually use threads
TSAN builds did not explicitly enable MULTITHREAD_SUPPORT_ENABLED, so they
relied on the CMake default. This adds -DMULTITHREAD_SUPPORT_ENABLED=ON to
both TSAN jobs and expands the CI matrix to cover threading on/off with
Debug/Release for Linux, macOS, and Windows MSVC. Also excludes async-dependent
.chai tests from non-threaded builds and adds a Threading_Config_Test to verify
the threading configuration is correct in both modes.
Co-Authored-By: Claude Opus 4.6 (1M context) [email protected]

Files

 .github/workflows/ci.yml            | 19 +++++++++++--------
 CMakeLists.txt                      | 13 +++++++++++++
 unittests/threading_config_test.cpp | 26 ++++++++++++++++++++++++++
 3 files changed, 50 insertions(+), 8 deletions(-)

Closes #666

Triggered by @lefticus.