Harden sglang sync workflow against PR token restrictions

[Copilot]

The Check for sglang-kt updates job was failing when repository policy disallowed PR creation via GITHUB_TOKEN. This change keeps the sync check usable in restricted repos while preserving automated PR behavior when elevated credentials are available.

• Token strategy

  • Introduce SYNC_PR_TOKEN with fallback: SGLANG_SYNC_TOKEN (PAT) → GITHUB_TOKEN.
  • Use the same token for checkout and create-pull-request to avoid mixed-auth behavior.

• Failure handling

  • Make PR creation non-fatal (continue-on-error: true) for update-check runs.
  • Add explicit warning output when PR creation is blocked, with actionable guidance to configure SGLANG_SYNC_TOKEN.

• Workflow scope

  • Changes are isolated to .github/workflows/sync-sglang-submodule.yml; no application/runtime code paths were modified.

env:
  SYNC_PR_TOKEN: ${{ secrets.SGLANG_SYNC_TOKEN || secrets.GITHUB_TOKEN }}

- uses: peter-evans/create-pull-request@v6
  id: create_pr
  continue-on-error: true
  with:
    token: ${{ env.SYNC_PR_TOKEN }}