feat(evals): link Datadog experiments to runtime traces

[mehulsonowal]

Summary

Eve evals can already observe the runtime trace contexts for every participating session, but the Datadog reporter drops those references when it submits Experiment rows. This change records a content-free, namespaced eveRuntimeTraceLinks array in each row's metadata so Datadog can resolve the canonical runtime traces at query time without changing trace IDs or parentage. Repeated contexts are deduplicated and a primary-session context wins when the same trace/span pair appears more than once.

This is the producer PR for the cross-repository query-time trace-expansion work. It does not pre-create Experiment spans, copy prompts or outputs into link metadata, or change runtime instrumentation.

Related draft PRs:

• Datadog API: https://github.com/ddoghq/dd-source/pull/89477
• Datadog UI: https://github.com/ddoghq/web-ui/pull/12362

Validation

• pnpm --filter eve build:compiled
• pnpm --filter eve exec vitest run --config vitest.unit.config.ts src/evals/runner/reporters/datadog.test.ts --reporter=verbose — 14 tests passed
• pnpm exec oxlint packages/eve/src/evals/runner/reporters/datadog.ts packages/eve/src/evals/runner/reporters/datadog.test.ts — no warnings or errors
• pnpm --filter eve typecheck
• GitHub core CI: lint, typecheck, unit, integration (Ubuntu/Windows), and scenario checks pass. test-tui and secret-backed E2E/Vercel jobs fail on this fork PR because GitHub withholds AI_GATEWAY_API_KEY and Vercel credentials; the logs report MODEL_CALL_FAILED: AI Gateway received no credentials before exercising this change.

Checklist

• This change was requested or approved by a maintainer
• I ran the relevant checks from CONTRIBUTING.md
• I added tests and documentation where relevant
• I added a changeset if this touches the published eve package
• DCO sign-off passes for every commit (git commit --signoff)