fix(telemetry): resolve clang-tidy include and modernize-use-auto CI errors

- Replace unused observer_result.h with nostd/shared_ptr.h and
  nostd/unique_ptr.h in MetricsRegistry.h (misc-include-cleaner)
- Use auto for cast initializations in MetricsRegistry.cpp and
  ValidationTracker test (modernize-use-auto)
- Run rename scripts: rippled -> xrpld in comments and docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/tests/libxrpl/telemetry/ValidationTracker.cpp

@@ -178,7 +178,7 @@ TEST_F(ValidationTrackerTest, MaxPendingEventsTrimming)
     for (std::size_t i = 0; i < kCount; ++i)
     {
         auto const hash = makeHash(i + 1);
-        LedgerIndex const seq = static_cast<LedgerIndex>(i + 1);
+        auto const seq = static_cast<LedgerIndex>(i + 1);
         tracker_.recordOurValidation(hash, seq);
         tracker_.recordNetworkValidation(hash, seq);
     }