[CI] Force HTTP/1.1 for runtime Git installs

[khluu]

Summary

• force Git's smart protocol to use HTTP/1.1 in the H200 Hybrid and V1 Core/KV/Metrics steps
• keep the workaround scoped to the jobs that install source dependencies from GitHub at runtime
• prevent HTTP/2 upload-pack 401 responses from turning into interactive username-prompt timeouts

Root cause

The failing H200 jobs can fetch public GitHub refs, but Git 2.43/libcurl 8.5 in the CI image sends the upload-pack POST over HTTP/2 and receives HTTP 401. Git then prompts for a username and the job hangs until its 65/80-minute timeout. Host-side Git on the same machines succeeds, so checkout and host egress are not the failing boundary.

The same CI image reproduces the protocol split exactly:

• forced HTTP/2: exit 128, fatal: expected flush after ref listing
• GIT_CONFIG_COUNT=1, GIT_CONFIG_KEY_0=http.version, GIT_CONFIG_VALUE_0=HTTP/1.1: public tag lookup succeeds

Representative failures:

• Hybrid shard 1: https://buildkite.com/vllm/ci/builds/86957#01a06435-5c3a-4e5f-97b7-3ba97e89b532
• Hybrid shard 2: https://buildkite.com/vllm/ci/builds/86957#01a06435-5c3a-4a73-b38c-04d7bb702d3e
• V1 Core/KV/Metrics: https://buildkite.com/vllm/ci/builds/86957#01a06435-5c39-42aa-ae00-07e4657ef9fc

Duplicate work

I searched open vLLM PRs and issues for http.version, HTTP/1.1, expected flush after ref listing, could not read Username, and the Mamba/causal-conv1d install path. I found no open PR addressing this CI transport failure.

Validation

• Reproduced forced HTTP/2 failure and HTTP/1.1 success in the same current vLLM CI image on h200-ci-4
• Parsed both modified YAML files through the ci-infra Step schema and asserted the exact environment mapping
• pre-commit run --hook-stage pre-commit --files .buildkite/test_areas/models_language.yaml .buildkite/test_areas/misc.yaml passed
• git diff --check passed
• Model evaluation: not applicable; this changes CI transport configuration only

This contribution was prepared with AI assistance and reviewed line by line by the human submitter.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[khluu]

Targeted exact validation is running in Buildkite #86981 at head 6912c9605358f9af67ddc1f60bb7563cd0ea2758, using the real fork branch and PR metadata with only:

• language-models-tests-hybrid (both H200 shards)
• v1-core-kv-metrics (H200 V1 lane)

Bootstrap passed and the three exact jobs rendered successfully. Their terminal results—especially the runtime Git installs—are the pre-merge gate.

[khluu]

Exact validation is terminal green in Buildkite #86981 at head 6912c9605358f9af67ddc1f60bb7563cd0ea2758.

• H200 V1 Core + KV + Metrics passed its full lane, including the 80-test suite and the formerly hanging runtime lm-eval Git clone/install. The clone resolved in seconds under the process-scoped HTTP/1.1 override, and the final lm-eval test passed.
• H200 Language Models Hybrid shard 1 passed 15 passed, 2 skipped after both runtime dependency fetches/builds succeeded.
• H200 Language Models Hybrid shard 2 passed 22 passed, 24 deselected after the runtime fetches succeeded.

This directly covers all three main-CI failure signatures fixed by this PR. The PR is now merged as 5e4e927b5e3a1cc7c1debe11e503a78ea241689f; post-merge main #86991 has selected the exact V1 and Hybrid lanes and they are currently running.