refactor(telemetry): add detachInPlace() helpers to remove hand-rolled SpanGuard re-detach idiom

Add two free functions in xrpl::telemetry that wrap the required
"detach a live SpanGuard held in a container" idiom so call sites write
one line instead of the make_shared/emplace rebuild by hand:

  void                detachInPlace(std::optional<SpanGuard>&);
  std::shared_ptr<SpanGuard> detachInPlace(std::shared_ptr<SpanGuard>);

Both are no-ops on an empty/null/inactive guard. The #else branch adds
matching inline no-op stubs so callers compile with telemetry disabled.
Pure API addition, no behavior change; consumer call sites are rewritten
on phase4 in a follow-up.

The unit tests for these helpers land on phase2, where the telemetry
test module (in-memory-exporter harness + SpanGuardScope.cpp) exists —
mirroring how the detached()/rootSpan() tests were placed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

include/xrpl/telemetry/SpanGuard.h

@@ -164,6 +164,7 @@
 #include <cstdint>
 #include <exception>
 #include <memory>
+#include <optional>
 #include <string_view>
 
 namespace xrpl::telemetry {
@@ -426,6 +427,43 @@ public:
     operator bool() const;
 };
 
+// --- Detach-in-place helpers -----------------------------------------------
+
+/**
+ * Detach an active `std::optional<SpanGuard>` member in place.
+ *
+ * Equivalent to `guard.emplace(std::move(*guard).detached())`, which is the
+ * required idiom for detaching a live guard stored in an `optional` (move
+ * assignment is deleted because the underlying Scope cannot be re-scoped in
+ * place). No-op if `guard` is empty or already null.
+ *
+ * @param guard  The optional guard to detach. Must be called on the thread
+ *               that constructed the active guard inside it (same rule as
+ *               `SpanGuard::detached()`).
+ * @note Must run BEFORE any `captureContext()` snapshot is taken if the
+ *       caller also needs the pre-detach context — capture first, then call
+ *       this helper (same ordering rule `detached()` itself has).
+ */
+void
+detachInPlace(std::optional<SpanGuard>& guard);
+
+/**
+ * Detach an active `std::shared_ptr<SpanGuard>`, returning the detached
+ * guard as a new `shared_ptr`.
+ *
+ * Equivalent to
+ * `guard = std::make_shared<SpanGuard>(std::move(*guard).detached())`.
+ * No-op (returns the input unchanged) if `guard` is null or points at a
+ * null guard.
+ *
+ * @param guard  The guard to detach, taken by value (the caller's pointer
+ *               is consumed; assign the return value back).
+ * @return A `shared_ptr` to the detached guard (new allocation), or the
+ *         input pointer unchanged if it was null/inactive.
+ */
+[[nodiscard]] std::shared_ptr<SpanGuard>
+detachInPlace(std::shared_ptr<SpanGuard> guard);
+
 // ---------------------------------------------------------------------------
 // No-op stub (all inline, zero overhead, no OTel dependency)
 // ---------------------------------------------------------------------------
@@ -539,6 +577,19 @@ public:
     }
 };
 
+// --- Detach-in-place helpers (no-op stubs) ---------------------------------
+
+inline void
+detachInPlace(std::optional<SpanGuard>&)
+{
+}
+
+[[nodiscard]] inline std::shared_ptr<SpanGuard>
+detachInPlace(std::shared_ptr<SpanGuard> guard)
+{
+    return guard;
+}
+
 #endif  // XRPL_ENABLE_TELEMETRY
 
 }  // namespace xrpl::telemetry

src/libxrpl/telemetry/SpanGuard.cpp

@@ -321,6 +321,24 @@ SpanGuard::detached() &&
     return SpanGuard(std::make_unique<Impl>(std::move(s), Impl::Detached{}));
 }
 
+// ===== Detach-in-place helpers =============================================
+
+void
+detachInPlace(std::optional<SpanGuard>& guard)
+{
+    if (!guard || !*guard)
+        return;
+    guard.emplace(std::move(*guard).detached());
+}
+
+std::shared_ptr<SpanGuard>
+detachInPlace(std::shared_ptr<SpanGuard> guard)
+{
+    if (!guard || !*guard)
+        return guard;
+    return std::make_shared<SpanGuard>(std::move(*guard).detached());
+}
+
 // ===== Context capture =====================================================
 
 SpanContext