fix(telemetry): address Phase 2 code review findings

- Move node health attribute strings to compile-time constants in
  SpanNames.h (attr::nodeAmendmentBlocked, attr::nodeServerState)
- Add Tempo search filters for node health attributes
- Remove unnecessary .c_str() on strOperatingMode() return
- Add samplingRatio clamping test (values > 1.0 and < 0.0)
- Fix Task 2.3 status: delivered in Phase 1c, not Phase 2

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/telemetry/grafana/provisioning/datasources/tempo.yaml

@@ -106,3 +106,14 @@ datasources:
             operator: "="
             scope: span
             type: dynamic
+          # Phase 2: Node health filters (Task 2.8)
+          - id: node-amendment-blocked
+            tag: xrpl.node.amendment_blocked
+            operator: "="
+            scope: span
+            type: static
+          - id: node-server-state
+            tag: xrpl.node.server_state
+            operator: "="
+            scope: span
+            type: dynamic