feat(sampling): Support for OpenTelemetry consistent tracestate sampling

[MilanGarnier]

What Does This Do

Adds OpenTelemetry consistent probability-sampling behavior on top of the
ot tracestate parsing introduced by #12405.

• Validates and normalizes rv and th.
• Generates probability state for loaded agent rates and sampling rules.
• Preserves inherited randomness and unknown fields across overrides.
• Keeps limiter outcomes distinct from raw probability decisions.
• Adds focused parsing, propagation, continuation, and sampling tests.

Motivation

Make Java tracer sampling decisions interoperable with OpenTelemetry
consistent probability sampling and provide the state needed for downstream
tracestate propagation.

Additional Notes

This is the second PR in a two-PR stack and depends on #12405.

This PR covers propagation and sampling only. OTLP export will follow in a
separate change.

Validation after rebasing the stack onto the current master:

• Focused dd-trace-core propagation and sampling tests passed.
• :dd-trace-core:compileTraceAgentTestJava passed.
• :dd-trace-core:spotlessCheck and git diff --check passed.

Prior validation of the feature branch:

• Java parametric system tests passed with 669 tests passed and 38 skipped.
• Java default system tests passed all 11 tests.
• The full :dd-trace-core:test run passed 4,277 of 4,278 tests. The remaining
  timing-sensitive PendingTraceBufferTest case passed in isolation.

Contributor Checklist

• Format the title according to the contribution guidelines.
• Assign the required type: and comp: labels.
• Keep OTLP export outside this propagation-only change.

Jira ticket: APMAPI-2171

[datadog-datadog-prod-us1-2]

🎯 Code Coverage (details)
• Patch Coverage: 90.12%
• Overall Coverage: 57.54% (-1.49%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 9e2422a | Docs | View more details | Give us feedback!}

[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	14.77 s	14.59 s	[+0.4%; +2.0%] (maybe worse)
startup:insecure-bank:tracing:Agent	13.61 s	13.70 s	[-1.5%; +0.2%] (no difference)
startup:petclinic:appsec:Agent	17.01 s	16.85 s	[-0.1%; +2.0%] (no difference)
startup:petclinic:iast:Agent	16.56 s	16.91 s	[-6.3%; +2.1%] (no difference)
startup:petclinic:profiling:Agent	16.72 s	16.80 s	[-1.6%; +0.6%] (no difference)
startup:petclinic:sca:Agent	17.00 s	16.65 s	[+1.1%; +3.0%] (significantly worse)
startup:petclinic:tracing:Agent	15.77 s	15.65 s	[-5.1%; +6.7%] (unstable)

Commit: a1686278 · CI Pipeline · Benchmarking Platform UI

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Load and DaCapo benchmarks can be triggered manually in the GitLab pipeline. Results will appear in the Benchmarking Platform UI after completion.