fix: use Python 3.10 in Minimal Tests — matches pyproject.toml constraint

backend/pyproject.toml requires >=3.10, <3.11.1. Python 3.11 (3.11.15)
is incompatible. Use 3.10 to match the project constraint.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: abhizipstack

.github/workflows/core-backend-tests.yaml

@@ -22,7 +22,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        python-version: ["3.11"]
+        python-version: ["3.10"]
     defaults:
       run:
         working-directory: backend