[Bugfix][Core] Fail cleanly for unsupported DBO backends

[cyq1017]

Purpose

Fixes #54493.

Replace the DBO all2all backend assert with a ValueError so unsupported configurations fail consistently even when Python assertions are disabled. The existing supported-backend list and remediation guidance are preserved.

This change was prepared with assistance from OpenAI Codex and reviewed by the contributor.

Test Plan

python -m pytest --noconftest -o addopts= -q \
  tests/test_config.py::test_all2all_backend_has_portable_default \
  tests/test_config.py::test_ubatching_rejects_unsupported_all2all_backend
ruff check vllm/config/vllm.py tests/test_config.py
ruff format --check vllm/config/vllm.py tests/test_config.py

Test Result

2 passed, 15 warnings in 4.26s
All checks passed!
2 files already formatted

The warnings are from the source checkout version lookup and PyTorch JIT deprecations.

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
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• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model. (Not applicable.)

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @cyq1017.

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