[Unified Cache][9/N] add opt-in MLA load deduplication for Mooncake Linker (#39565)

Co-authored-by: Zhangheng <hzh0425@apache.org>

test/registered/unit/server_args/test_server_args.py

@@ -1995,6 +1995,25 @@ class TestSSLArgs(unittest.TestCase):
 
 
 class TestHiCacheArgs(unittest.TestCase):
+    def test_linker_mla_dedup_requires_mooncake_linker(self):
+        for enabled, linker, backend in (
+            (False, False, "mooncake"),
+            (True, True, "mooncake"),
+            (True, False, "mooncake"),
+            (True, True, "mori"),
+        ):
+            with self.subTest(enabled=enabled, linker=linker, backend=backend):
+                args = self._make_args(
+                    enable_linker_mla_dedup=enabled,
+                    enable_unified_cache_external_linker=linker,
+                    unified_cache_external_linker_backend=backend,
+                )
+                if enabled and (not linker or backend != "mooncake"):
+                    with self.assertRaisesRegex(ValueError, "requires the Mooncake"):
+                        handle_hicache(args)
+                else:
+                    handle_hicache(args)
+
     def _make_args(self, **overrides) -> ServerArgs:
         # Not resolved: a dummy model path takes the pipeline's early return,
         # so `_handle_hicache` would never run. Its one prerequisite (the