Add ZTS (Thread Safety) to CI test matrix #18

FrankenPHP requires ZTS (Zend Thread Safety). We need to verify php-debugger works under ZTS.

What to do

1. Add --enable-zts build variants to tests.yml (at least PHP 8.4 × ZTS)
2. Fix any thread-safety issues that surface (missing TSRM macros, non-thread-local globals)
3. Verify all TSRM cache macros are present: ZEND_TSRMLS_CACHE_DEFINE(), ZEND_TSRMLS_CACHE_UPDATE()

Acceptance criteria

• CI runs tests with --enable-zts on at least one PHP version
• All tests that pass in NTS also pass in ZTS
• No thread-safety warnings/errors