[smg] Work around sglang's notorious orphan process problem

[slin1237]

sglang is notorious for getting stuck during model loading and leaving orphan child processes (TP workers) hanging around holding GPU memory. There's even a dedicated killall_sglang.sh script for this very reason.

This commit works around these issues in e2e tests:

• Kill entire process group on terminate() instead of just parent process, since sglang doesn't clean up its children when the parent dies
• Increase LAUNCH_STAGGER_DELAY from 5s to 10s because sglang can't handle multiple servers loading weights simultaneously without choking

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
• Follow the SGLang code style guidance.

Review Process

1. Ping Merge Oncalls to start the PR flow. See the PR Merge Process.
2. Get approvals from CODEOWNERS and other reviewers.
3. Trigger CI tests with comments (/tag-run-ci-label, /rerun-failed-ci, /tag-and-rerun-ci) or contact authorized users to do so.
4. After green CI and required approvals, ask Merge Oncalls to merge.

[gemini-code-assist]

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