[nexus] dynamically sync radio range circles with simulator parameters (#12987)

This commit exposes radio model parameters (path loss constant,
exponent, and sensitivity) and the minimum link request margin from
the Nexus simulator backend to the frontend.

Changes in backend:
- Expose constants in `RadioModel` and `Radio`.
- Add `GetRadioParameters` RPC to `simulation.proto` and implement it
  in gRPC and WASM bindings.
- Expose `OPENTHREAD_CONFIG_MLE_LINK_REQUEST_MARGIN_MIN` and
  `OPENTHREAD_CONFIG_MLE_PARTITION_MERGE_MARGIN_MIN` via the new RPC.

Changes in config:
- Set `OPENTHREAD_CONFIG_MLE_LINK_REQUEST_MARGIN_MIN` and
  `OPENTHREAD_CONFIG_MLE_PARTITION_MERGE_MARGIN_MIN` to 5 dB in
  `openthread-core-nexus-config.h`.

This allows the frontend to calculate and render circles dynamically.

tests/nexus/openthread-core-nexus-config.h

@@ -109,6 +109,8 @@
 #define OPENTHREAD_CONFIG_MLE_LINK_METRICS_INITIATOR_ENABLE 1
 #define OPENTHREAD_CONFIG_MLE_LINK_METRICS_SUBJECT_ENABLE 1
 #define OPENTHREAD_CONFIG_MLE_MAX_CHILDREN 128
+#define OPENTHREAD_CONFIG_MLE_LINK_REQUEST_MARGIN_MIN 5
+#define OPENTHREAD_CONFIG_MLE_PARTITION_MERGE_MARGIN_MIN 5
 #define OPENTHREAD_CONFIG_MLR_ENABLE 1
 #define OPENTHREAD_CONFIG_BACKBONE_ROUTER_MULTICAST_ROUTING_ENABLE 1
 #define OPENTHREAD_CONFIG_MULTICAST_DNS_AUTO_ENABLE_ON_INFRA_IF 1

tests/nexus/platform/nexus_grpc.cpp

@@ -44,6 +44,8 @@
 
 #include "nexus_logging.hpp"
 #include "nexus_node.hpp"
+#include "nexus_radio.hpp"
+#include "nexus_radio_model.hpp"
 #include "simulation.grpc.pb.h"
 #include "common/clearable.hpp"
 #include "common/code_utils.hpp"
@@ -269,6 +271,18 @@ public:
         return status;
     }
 
+    grpc::Status GetRadioParameters(grpc::ServerContext * /* aContext */,
+                                    const nexus::GetRadioParametersRequest * /* aRequest */,
+                                    nexus::GetRadioParametersResponse *aResponse) override
+    {
+        aResponse->set_path_loss_constant(RadioModel::kPathLossConstant);
+        aResponse->set_path_loss_exponent(RadioModel::kPathLossExponent);
+        aResponse->set_radio_sensitivity(Radio::kRadioSensitivity);
+        aResponse->set_mle_link_request_margin_min(OPENTHREAD_CONFIG_MLE_LINK_REQUEST_MARGIN_MIN);
+
+        return grpc::Status::OK;
+    }
+
 private:
     GrpcServer *mServerPtr;
 };

tests/nexus/platform/nexus_radio_model.cpp

@@ -38,9 +38,6 @@ namespace Nexus {
 
 int16_t RadioModel::CalculateRssi(const Node &aTxNode, const Node &aRxNode)
 {
-    static constexpr double kPathLossConstant = 40.0;
-    static constexpr double kPathLossExponent = 20.0;
-
     // Simple path loss model
     // RSSI = TxPower - PathLoss
     // PathLoss = kPathLossExponent * log10(distance) + kPathLossConstant

tests/nexus/platform/nexus_radio_model.hpp

@@ -43,6 +43,9 @@ class Node;
 class RadioModel
 {
 public:
+    static constexpr double kPathLossConstant = 40.0;
+    static constexpr double kPathLossExponent = 20.0;
+
     RadioModel(void) = delete;
 
     /**

tests/nexus/platform/nexus_wasm.cpp

@@ -46,6 +46,8 @@
 
 #include "nexus_core.hpp"
 #include "nexus_node.hpp"
+#include "nexus_radio.hpp"
+#include "nexus_radio_model.hpp"
 #include "mac/mac.hpp"
 #include "thread/mle.hpp"
 #include "thread/neighbor_table.hpp"
@@ -225,6 +227,15 @@ EMSCRIPTEN_BINDINGS(nexus_simulator)
 
     function("pollEvent", optional_override([]() -> val { return WasmManager::Get().PollEvent(); }));
 
+    function("getRadioParameters", optional_override([]() -> val {
+                 val params = val::object();
+                 params.set("pathLossConstant", RadioModel::kPathLossConstant);
+                 params.set("pathLossExponent", RadioModel::kPathLossExponent);
+                 params.set("radioSensitivity", Radio::kRadioSensitivity);
+                 params.set("mleLinkRequestMarginMin", OPENTHREAD_CONFIG_MLE_LINK_REQUEST_MARGIN_MIN);
+                 return params;
+             }));
+
     function("getNow", optional_override([]() -> uint32_t { return Core::Get().GetNow().GetValue(); }));
 
     function("stepSimulation", optional_override([](uint32_t aElapsedMs) { Core::Get().AdvanceTime(aElapsedMs); }));

tests/nexus/platform/simulation.proto

@@ -212,6 +212,20 @@ service NexusService {
    * Joins a node to a network.
    */
   rpc JoinNetwork(JoinNetworkRequest) returns (JoinNetworkResponse);
+
+  /**
+   * Retrieves the radio model parameters.
+   */
+  rpc GetRadioParameters(GetRadioParametersRequest) returns (GetRadioParametersResponse);
+}
+
+message GetRadioParametersRequest {}
+
+message GetRadioParametersResponse {
+  float path_loss_constant = 1;
+  float path_loss_exponent = 2;
+  float radio_sensitivity = 3;
+  int32 mle_link_request_margin_min = 4;
 }
 
 message StreamRequest {