fix(fault_manager): explicit ctor for LockedSubscriptionGuard (C++20 aggregate)

Rolling's gcc 14 / libstdc++ enforces the C++20 wording of [dcl.init.aggr]
that disqualifies a class as an aggregate when it has any user-declared
constructors - including ``= delete`` ones (rule tightened from C++17's
"user-provided" to C++20's "user-declared"). Even with
CMAKE_CXX_STANDARD=17 set, the libstdc++ headers/checks behave as C++20
on rolling, breaking ``LockedSubscriptionGuard{&mtx, sub, cg}`` because
the only candidates are the deleted copy/move constructors.

Add an explicit 3-arg constructor so brace-init resolves to direct-init
regardless of aggregate semantics across distros (Jazzy/Humble C++17 and
Rolling effective-C++20). Reproduces locally with ``g++ -std=c++20``.

Author: bburda

src/ros2_medkit_fault_manager/src/snapshot_capture.cpp

@@ -40,6 +40,18 @@ struct LockedSubscriptionGuard {
   rclcpp::GenericSubscription::SharedPtr subscription;
   rclcpp::CallbackGroup::SharedPtr callback_group;
 
+  // Explicit constructor instead of relying on aggregate initialization:
+  // C++20 [dcl.init.aggr] disqualifies a class as an aggregate if it has
+  // any user-declared constructors, including ``= delete`` ones (the rule
+  // tightened from "user-provided" in C++17 to "user-declared" in C++20).
+  // Rolling's gcc 14 / libstdc++ enforces the C++20 wording even when
+  // CMAKE_CXX_STANDARD is 17, so brace-init ``LockedSubscriptionGuard{
+  // &mtx, sub, cg}`` would fail to find a matching constructor on rolling.
+  // An explicit constructor sidesteps the aggregate-init rules entirely.
+  LockedSubscriptionGuard(std::mutex * m, rclcpp::GenericSubscription::SharedPtr s, rclcpp::CallbackGroup::SharedPtr cg)
+    : mtx(m), subscription(std::move(s)), callback_group(std::move(cg)) {
+  }
+
   ~LockedSubscriptionGuard() {
     if (!subscription && !callback_group) {
       return;