DectrisSimplonClient: separate connections for commands and status reads

Commands and configuration writes (initialize, disarm, threshold change) can
take minutes. With one shared connection a status poll waited behind them;
now commands/config writes go through one kept-alive client and status/config
reads through another, each with its own mutex and fixed timeouts.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/DectrisSimplonClientTest.cpp

@@ -4,13 +4,16 @@
 #include <catch2/catch_all.hpp>
 #include <httplib.h>
 #include <nlohmann/json.hpp>
+#include <atomic>
+#include <chrono>
 #include <mutex>
 #include <set>
 #include <thread>
 
 #include "../detector_control/DectrisSimplonClient.h"
 
-// A minimal SIMPLON server: the detector state and error list, and a disarm command that fails.
+// A minimal SIMPLON server: the detector state and error list, a disarm command that fails and a
+// cancel command that takes 2 s.
 TEST_CASE("DectrisSimplonClient_MockServer", "[detector]") {
     httplib::Server server;
     std::mutex m;
@@ -34,6 +37,13 @@ TEST_CASE("DectrisSimplonClient_MockServer", "[detector]") {
         res.set_content("disarm refused", "text/plain");
     });
 
+    std::atomic<bool> cancel_running = false;
+    server.Post("/detector/api/1.8.0/command/cancel", [&](const httplib::Request &, httplib::Response &res) {
+        cancel_running = true;
+        std::this_thread::sleep_for(std::chrono::seconds(2));
+        res.set_content("", "text/plain");
+    });
+
     const int port = server.bind_to_any_port("127.0.0.1");
     REQUIRE(port > 0);
     struct ServerThread {
@@ -53,6 +63,15 @@ TEST_CASE("DectrisSimplonClient_MockServer", "[detector]") {
 
     CHECK(client.GetErrorStatus() == R"(["overheating"])");
 
+    // A long command does not hold up status polls
+    std::thread cancel([&client] { client.CancelAcquisition(); });
+    while (!cancel_running)
+        std::this_thread::sleep_for(std::chrono::milliseconds(1));
+    auto t0 = std::chrono::steady_clock::now();
+    CHECK(client.GetState() == SimplonState::Idle);
+    CHECK(std::chrono::steady_clock::now() - t0 < std::chrono::milliseconds(500));
+    cancel.join();
+
     // Failures carry the HTTP status and the body the DCU sent
     {
         std::lock_guard lg(m);