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) <noreply@anthropic.com>
