[router] Improve cleanup logic

[ByronHsu]

The current sglang_router.sglang_server has overcomplicated logic of process cleanup

1. Replace force kill with terminate because sglang server handles graceful termination internally. It is safer and more predictable too.
2. Remove exception catching because it is already handled by python internal.

The current test_launch_server has a bug

1. We shutdown the router server per class, but it should be per test case. The test passed just by luck because python ignores the port occupation error.

Depends on #2410