test(moe): add GLM MegaMoE validation coverage

Add parser coverage for --moe-a2a-backend=megamoe and local unit coverage for MegaMoE gating, GLM fast-path routing, fallback routing, and sidecar weight preservation.

Use monkeypatch-style tests so the forward and weight-layout checks do not require loading GLM 5.2 or importing a real deep_gemm runtime.

Constraint: tests must not stage or depend on local skill/doc artifacts.

Feature-flag: --moe-a2a-backend=megamoe.

Conflict-hotspots: test/registered/unit/server_args/test_server_args.py, test/registered/unit/moe/test_glm_megamoe.py.

Scope-risk: runtime GPU e2e still needs target Blackwell/DeepGEMM environment.

Tested: PYTHONPYCACHEPREFIX=/private/tmp/sglang_pycache python3 -m py_compile test/registered/unit/server_args/test_server_args.py test/registered/unit/moe/test_glm_megamoe.py.

Tested: git diff --check.

Not-tested: PYTHONPATH=python python3 -m pytest test/registered/unit/moe/test_glm_megamoe.py -q; local Python has no pytest.

Not-tested: PYTHONPATH=python python3 -m unittest test.registered.unit.moe.test_glm_megamoe -v; local Python has no torch.

Not-tested: GLM 5.2 MegaMoE GPU e2e; local environment lacks target runtime and hardware.

test/registered/unit/server_args/test_server_args.py

@@ -1,4 +1,5 @@
 import json
+import os
 import tempfile
 import unittest
 from unittest.mock import MagicMock, patch
@@ -195,6 +196,46 @@ def test_nsa_index_skip_topk_offset_zero_means_unset():
     assert override_args == {"index_topk_freq": 4}
 
 
+def test_megamoe_a2a_backend_parser_sets_ep_to_tp():
+    import argparse
+
+    from sglang.srt.environ import envs
+
+    field = envs.SGLANG_OPT_FIX_MEGA_MOE_MEMORY
+    backup_present = field.name in os.environ
+    backup_value = os.environ.get(field.name)
+    backup_set_to_none = field._set_to_none
+    os.environ.pop(field.name, None)
+    field._set_to_none = False
+    field._cache_valid = False
+
+    try:
+        parser = argparse.ArgumentParser()
+        ServerArgs.add_cli_args(parser)
+        raw_args = parser.parse_args(
+            [
+                "--model-path",
+                "dummy",
+                "--tp-size",
+                "4",
+                "--moe-a2a-backend",
+                "megamoe",
+            ]
+        )
+        args = ServerArgs.from_cli_args(raw_args)
+        assert args.moe_a2a_backend == "megamoe"
+        assert args.tp_size == 4
+        assert args.ep_size == 4
+        assert field.get() is True
+    finally:
+        if backup_present:
+            os.environ[field.name] = backup_value
+        else:
+            os.environ.pop(field.name, None)
+        field._set_to_none = backup_set_to_none
+        field._cache_valid = False
+
+
 class TestLoadBalanceMethod(unittest.TestCase):
     def test_non_pd_defaults_to_round_robin(self):
         server_args = ServerArgs(model_path="dummy", disaggregation_mode="null")