[CI] Validate registered test __main__ before /rerun-test dispatch

[nvpohanh]

[by Codex]

Summary

This makes /rerun-test apply the same registered-test __main__ sanity check that the normal suite path already uses.

Changes

• load python/sglang/test/ci/ci_register.py from the slash-command handler
• validate resolved registered test files with collect_tests(..., sanity_check=True) before dispatch
• add a regression test covering the missing-__main__ rejection path

Validation

• python3 -m unittest scripts/ci/utils/test_slash_command_handler.py
• BLACK_NUM_WORKERS=1 SKIP=no-commit-to-branch /home/pohanh/.local/bin/pre-commit run --all-files --show-diff-on-failure

Related

• follow-up to PR [CI] Add entrypoint to hc_combine test for standalone execution #37406
• addresses TER issue [CI] Test Escape Report: /rerun-test did not enforce registered test __main__ validation #37526

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CI States

Latest PR Test (Base): ❌ Run #33587614149
Latest PR Test (Extra): ❌ Run #33587613978
Latest PR Test (AMD ROCm 7.2): ❌ Run #33587614079

[nvpohanh]

should we use global variable for this or just LRU cache?