[sglang] feat: retires sglang spmd mode in the codebase

[PeterSH6]

What does this PR do?

Retires the legacy SGLang SPMD rollout path and makes async/server mode the only supported backend for SGLang. The PR removes the old SGLangRollout class, its helpers, tests, and recipes, and updates all docs, scripts, and CI references so they speak only to the async HTTP adapter (ServerAdapter).

Checklist Before Starting

• Search for similar PRs. Paste at least one query link here: N/A (follow-up to the vLLM SPMD removal).
• Format the PR title as [sglang, rollout, trainer, recipe, ci, doc] refactor: remove SGLang SPMD rollout

Test

API and Usage Example

Same as vLLM: configs/scripts must set actor_rollout_ref.rollout.mode=async and rely on the HTTP server. Example:

python -m verl.trainer.main_ppo \
  ... \
  actor_rollout_ref.rollout.name=sglang \
  actor_rollout_ref.rollout.mode=async \
  ...

Design & Code Changes

• Deleted the SGLangRollout class and associated helpers from verl/workers/rollout/sglang_rollout/sglang_rollout.py, keeping only the async ServerAdapter. Cleared its registry entries, configs, and guards the same way as the vLLM PR.
• Removed SGLang SPMD-specific tests (tests/workers/rollout/test_sglang_*) and CI steps in .github/workflows/sgl.yml, plus any lint exclusions that referenced those files.
• Updated recipes/examples/e2e scripts that referenced SGLang rollout to hardcode rollout.mode=async, drop sync branches, and set return_raw_chat (mirroring the vLLM cleanup).

Checklist Before Submitting

• Read the Contribute Guide.
• Apply pre-commit checks.
• Add / Update the documentation.
• Add unit or end-to-end test(s) to the CI workflow to cover all the code. If not feasible, explain why: Removed the obsolete SGLang SPMD jobs; async workflows remain covered.
• Once your PR is ready for CI, notify the ci-request channel (or Feishu group).

[gemini-code-assist]

Code Review

This pull request successfully retires the legacy SGLang SPMD mode, streamlining the codebase to exclusively use the async/server mode. The SGLangRollout class, along with its associated helper functions, tests, and configuration references, has been removed. This is a positive and significant refactoring effort that enhances maintainability and clarifies the supported SGLang backend. The changes are consistently applied across example scripts, test files, and core implementation, ensuring a clean transition to the new architecture.

[PeterSH6]

Previously retire vllm: #4411