fix: improve compatibility with older SGLang versions

[ForAxel]

Summary

Improve compatibility with SGLang releases that do not expose all of the argument aliases and router options expected by the latest Slime integration.

Changes

• Skip SGLang parallelism aliases when neither the current nor legacy argument name is available, while continuing to synchronize aliases that are provided by the installed SGLang version.
• Set disable_health_check only when the installed sglang-router exposes that option, preventing unsupported dynamic attributes from being forwarded to the native Router constructor.

Motivation

SGLang argument and router APIs vary between releases. Slime previously assumed that at least one name in every argument alias pair and the disable_health_check router option were always available. With releases that do not provide those attributes, validation or router startup could fail before rollout initialization.

The new capability checks preserve the existing behavior for versions that support these APIs and safely skip unavailable options for versions that do not.

Testing

PYTHONPATH=. pytest -q tests/utils/test_sglang_arguments.py tests/test_megatron_argument_validation.py

Result: 20 passed.