[gcp-sphinx-docfx-yaml] Restore unit test sessions for Python 3.8-3.9 and 3.11-3.14 #16176
Description
Unit tests are currently restricted to Python 3.10 to bypass environment-specific setup issues encountered during migration. We need to verify the test suite and resolve any dependency conflicts to re-enable the full support matrix from 3.8 through 3.14.