[PD-Disagg] Fully support external DP dispatch w/ PD-disaggregation mode. (#19268)

Co-authored-by: Ratish P <114130421+ratish1@users.noreply.github.com>

sgl-model-gateway/bindings/python/src/sglang_router/router_args.py

@@ -18,6 +18,7 @@ class RouterArgs:
 
     # PD-specific configuration
     mini_lb: bool = False
+    test_external_dp_routing: bool = False
     pd_disaggregation: bool = False  # Enable PD disaggregated mode
     prefill_urls: List[tuple] = dataclasses.field(
         default_factory=list
@@ -360,6 +361,11 @@ class RouterArgs:
             action="store_true",
             help="Enable MiniLB",
         )
+        pd_group.add_argument(
+            f"--{prefix}test-external-dp-routing",
+            action="store_true",
+            help="(MiniLB only) Randomly assign routed_dp_rank / disagg_prefill_dp_rank per request and verify the response dp_rank matches.",
+        )
         pd_group.add_argument(
             f"--{prefix}pd-disaggregation",
             action="store_true",