chore: add Python 3.14 classifier and testing

Author: stainless-app[bot]

pyproject.toml

@@ -26,6 +26,7 @@ classifiers = [
   "Programming Language :: Python :: 3.11",
   "Programming Language :: Python :: 3.12",
   "Programming Language :: Python :: 3.13",
+  "Programming Language :: Python :: 3.14",
   "Operating System :: OS Independent",
   "Operating System :: POSIX",
   "Operating System :: MacOS",