[refactor] Resolve config declarations onto server_args at the end of __post_init__ (#30297)

test/registered/mock_model/test_self_unit_install.py

@@ -17,6 +17,10 @@ register_amd_ci(est_time=60, suite="extra-a-test-1-gpu-small-amd")
 
 
 def _make_server_args(*, sampling_backend: str) -> SimpleNamespace:
+    # The install gate reads the resolved backend from the flags tier.
+    from sglang.srt.runtime_context import get_flags
+
+    get_flags().sampling_backend = sampling_backend
     return SimpleNamespace(sampling_backend=sampling_backend)