[patch] support applying SGLang patch inside a git repo

[FrankLeeeee]

Motivation

scripts/apply_sglang_spec_capture_patch.sh silently does nothing when the installed SGLang lives inside a git checkout. It runs git apply from the site-packages directory. If that directory is nested in a repository, git resolves patch paths against the repository top level and skips every file outside the current subdirectory, then exits 0.

This happens if the user's env is inside specforge repo, e.g. users can call uv venv -p 3.12 in specforge repo and keep the virtual env there.

Modifications

• Route all git apply calls in the script through one helper that sets GIT_CEILING_DIRECTORIES to the parent of the SGLang root. This stops repository discovery, so git treats site-packages as a plain directory and patch paths resolve correctly.
• Add a regression test that places the fake site-packages inside a fresh git repository and checks that the patch applies, that a second run is idempotent, and that --reverse restores the original files. The test fails on the old script and passes on the new one.

Related Issues

Accuracy Test

Benchmark & Profiling

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
• For reviewers: If you haven't made any contributions to this PR and are only assisting with merging the main branch, please remove yourself as a co-author when merging the PR.
• Please feel free to join our Slack channel at https://sgl-fru7574.slack.com/archives/C09784E3EN6 to discuss your PR.