Always pass a non-null SpanRef to withSpan closures

[lbwexler]

Summary

• TraceService.withSpan now supplies a shared SpanRef.NOOP (backed by Span.invalid / Scope.noop()) when tracing is disabled, so closures always receive a non-null SpanRef.
• Removes the need for ?. null-safe calls on span at every instrumented call site — updated JSONClient, BaseProxyService, docs, and examples.
• ObservedRun.run carries the same non-null guarantee via activeSpan = SpanRef.NOOP default.

Test plan

• ./gradlew assemble (compileGroovy passes locally).
• Exercise a traced code path with tracing disabled and confirm span.setTag / span.setHttpStatus are safe no-ops.
• Exercise the same path with xhTraceConfig.enabled = true and confirm spans export normally.

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