Optimize CI: test only Python 3.9 and 3.14 on PRs

Configure GitHub Actions to run tests on only the minimum and maximum
supported Python versions (3.9 and 3.14) for pull requests, while still
testing all versions (3.9-3.14) on pushes to master.

This reduces CI time and resource usage for PRs while maintaining full
coverage on the main branch.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: tobixen

.github/workflows/tests.yaml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
+        python: ${{ github.event_name == 'pull_request' && fromJSON('["3.9", "3.14"]') || fromJSON('["3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]') }}
     services:
       baikal:
         image: ckulka/baikal:nginx