fix(telemetry): restore injectCurrentContextToProtobuf decl; re-parent open/establish spans under round; correct childSpan names

Post-review fixes for the SpanGuard scope-leak change:
- SpanGuard.h: restore the injectCurrentContextToProtobuf real-class
  declaration, its #else no-op stub, and the protocol::TraceContext
  forward-declaration dropped during the 3->4 union merge (compile break).
- Re-parent consensus.phase.open and consensus.establish under the round
  span via a new RCLConsensus::Adaptor::roundSpanContext() accessor: the
  round span is now detached, so ambient parenting no longer works; the
  phase spans link explicitly to the captured round context. csf::Peer
  gains a matching no-op accessor so the generic engine still compiles.
- Switch five childSpan(op::X, ctx) sites to the full consensus::span::X
  constants: childSpan(name, ctx) takes the name verbatim, so the suffix-
  only op:: constants emitted short, non-dotted span names.
- TestTelemetry mock: add getConsensusTraceStrategy() override (base pure
  virtual introduced on this branch) so the mock is not abstract.
- validate(): rewrite the trace_context comment to drop attack-surface
  framing and the PR-discussion reference.

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

src/tests/libxrpl/telemetry/SpanGuardScope.cpp

@@ -135,6 +135,17 @@ public:
         return true;
     }
 
+    /**
+     * @return A fixed strategy label; the scope tests do not exercise
+     * deterministic trace-id correlation, so any stable value works.
+     */
+    [[nodiscard]] std::string const&
+    getConsensusTraceStrategy() const override
+    {
+        static std::string const kStrategy{"none"};
+        return kStrategy;
+    }
+
     opentelemetry::nostd::shared_ptr<opentelemetry::trace::Tracer>
     getTracer(std::string_view name) override
     {