Fix CI for GPT-5 Codex (#2314): exclude occ_tests from race split, run serial

[Pray4Love1]

• Excludes occ_tests from split/race groups (wasmvm+cgo crashes with SIGBUS)
• Adds dedicated serial test-occ target without -race
• Adds debug printing of split groups
• Uses checkout fetch-depth: 0 + fetch-tags: true to fix git describe errors

[github-actions[bot]]

We truly appreciate your contribution and the time you’ve invested in this PR. Before we can merge it, we’d love your help addressing the remaining feedback or sharing your perspective. If we don’t hear back within 2 day(s), this PR will close automatically — but don’t worry, you can reopen it anytime when you’re ready to continue.