fix(ci): isolate model-loading fork tests

[mday-io]

Description

Model-loading fork tests intermittently segfault in Python 3.9 CI when they share a pytest process. Forked workers can inherit native state left by earlier tests.

Add a fork_isolated marker and run test_parallel_load and test_parallel_load_multi_repo in separate pytest processes in make fast-test, make slow-test, and make cicd-test.

Test Plan

• Ran each fork test 30 times in a separate process on Linux with Python 3.9 and 3.12. All 120 runs passed. Runs of the whole test file also passed, so these checks do not prove the intermittent crash is eliminated.
• Verified that the revised batches select the same tests with no omissions or duplicates.
• Ran make style. Ran the full make fast-test command on the final PR commit on macOS with Python 3.12.13. All six pytest batches passed, with 2,860 tests passed in total, including both fork tests in separate processes.
• GitHub CI passed the style and CI/CD tests on Python 3.9 through 3.13 and Windows. All checks that ran passed. Cloud engine tests and VS Code end-to-end tests were skipped.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO