Add Vercel sandbox environment support

[taivu1998]

Summary

This PR adds first-class Vercel Sandbox support as a new isolated RLM environment.

What’s included

• Add VercelREPL in rlm/environments/vercel_repl.py
• Register environment="vercel" in the environment factory and EnvironmentType
• Add a vercel optional dependency extra in pyproject.toml
• Add README guidance for Vercel Sandboxes and clarify the distinction between:
  • backend="vercel" for AI Gateway
  • environment="vercel" for Vercel Sandboxes
• Add examples/vercel_repl_example.py
• Add focused mocked test coverage for the new environment

Design

The implementation follows the existing isolated-environment pattern already used by Modal/Prime/Daytona/E2B:

• Create a Vercel Sandbox with explicit python3.13
• Start a lightweight HTTP broker inside the sandbox
• Poll broker requests from the host
• Forward LM requests through LMHandler
• Return responses back into the sandbox
• Preserve REPL locals across iterations within a single completion

Notable v1 choices:

• persistent=True is explicitly unsupported
• rlm_query(...) and rlm_query_batched(...) are exposed in the sandbox as thin aliases over the llm_query variants
• The broker and sandbox helper path use stdlib HTTP utilities instead of Flask/requests inside the sandbox, which keeps Vercel startup simpler
• Broker HTTP error payloads are preserved so timeout/error messages surface cleanly

Files changed

• rlm/environments/vercel_repl.py
• rlm/environments/__init__.py
• rlm/core/types.py
• pyproject.toml
• README.md
• examples/vercel_repl_example.py
• tests/test_vercel_repl.py
• tests/test_imports.py
• tests/test_multi_turn_integration.py

Verification

Ran:

• .venv/bin/ruff check rlm/environments/vercel_repl.py tests/test_vercel_repl.py tests/test_imports.py tests/test_multi_turn_integration.py examples/vercel_repl_example.py
• .venv/bin/ruff format --check rlm/environments/vercel_repl.py tests/test_vercel_repl.py tests/test_imports.py tests/test_multi_turn_integration.py examples/vercel_repl_example.py
• .venv/bin/python -m compileall rlm/environments/vercel_repl.py examples/vercel_repl_example.py tests/test_vercel_repl.py
• .venv/bin/pytest tests/test_vercel_repl.py tests/test_imports.py tests/test_types.py tests/test_multi_turn_integration.py::TestPersistentModeValidation::test_vercel_environment_raises_error

Result:

• 63 passed, 4 skipped

Notes

I did not run the full socket-heavy integration suite in this environment because localhost socket binds for LMHandler are blocked here with PermissionError: [Errno 1] Operation not permitted. The new Vercel-specific coverage and public-surface checks are passing.

[taivu1998]

Hi @alexzhang13, could you help review it when you have time? Thanks!