Fix SpanStack active reference corruption

[morrisonlevi]

Description

This PR is stacked on #3851 so that needs to merge first.

This test demonstrates that the active span can be corrupted by turning it into a reference. This may be the cause of a customer crash identified by a crash report.

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[datadog-official]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 60.68% (+0.04%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 189b253 | Docs | Datadog PR Page | Give us feedback!}

[pr-commenter]

Benchmarks [ tracer ]

Benchmark execution time: 2026-05-01 17:05:10

Comparing candidate commit ae8cb30 in PR branch levi/spanstack-active-reference with baseline commit a51ed01 in branch levi/parent-refcount.

Found 0 performance improvements and 4 performance regressions! Performance is the same for 190 metrics, 0 unstable metrics.

scenario:SamplingRuleMatchingBench/benchRegexMatching1

• 🟥 execution_time [+50.086ns; +134.114ns] or [+3.484%; +9.330%]

scenario:SamplingRuleMatchingBench/benchRegexMatching2

• 🟥 execution_time [+66.091ns; +135.309ns] or [+4.612%; +9.442%]

scenario:SamplingRuleMatchingBench/benchRegexMatching3

• 🟥 execution_time [+80.106ns; +165.694ns] or [+5.531%; +11.440%]

scenario:SamplingRuleMatchingBench/benchRegexMatching4

• 🟥 execution_time [+44.296ns; +142.104ns] or [+3.035%; +9.736%]

[bwoebi]

Thanks for the reproducer!