Reduce CI matrix to one Python version per OS

Cuts test runners from 9 (3 OS x 3 Python) to 3 (3 OS x Python 3.11)
for faster, cheaper CI while maintaining cross-platform coverage.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: rajeee

.github/workflows/tests.yml

@@ -14,7 +14,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: ["3.10", "3.11", "3.12"]
+        python-version: ["3.11"]
 
     steps:
       - name: Checkout repository