[Bugfix][Frontend] Allow selecting Uvicorn HTTP implementation

[navaneethkrishnansuresh]

Fixes #57203.

Adds --http to select Uvicorn’s HTTP/1.1 implementation. h11 preserves h2c-upgrade request bodies.

No existing open PR addresses this issue.

Tests:

• VLLM_TARGET_DEVICE=cpu .venv/bin/python -m pytest tests/entrypoints/launchers/test_cli_args.py tests/entrypoints/launchers/test_dp_supervisor.py -k 'http_implementation or start_server_no_log_config_when_no_filter' -q — 5 passed
• Manual h2c request: auto returned 400; h11 returned 200

Model evaluation: not applicable; this is HTTP transport configuration only.

AI assistance used; the author reviewed the change and validation.

[claude]

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[navaneethkrishnansuresh]

Hi @NickLucche @aarnphm @AndreasKaratzas @DarkLight1337 @robertgshaw2-redhat gentle ping on this PR when you get a chance.

[mergify]

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

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork