🐛 [RUMS-6286] avoid fallback trace sampling in WebViews

[amortemousque]

Motivation

In WebViews, the native SDK owns the trace sampling decision. When that decision is temporarily unavailable, the Browser SDK currently falls back to its own sampling decision. The two decisions can differ and produce inconsistent tracing for the same mobile session.

This addresses RUMS-6286.

Changes

• Treat a missing native trace sampling decision as not sampled when the event bridge is active.
• Keep the Browser SDK sampling decision for non-WebView usage.
• Add unit coverage for XHR and Fetch requests.
• Add E2E coverage for an event bridge without a native sampling decision.

The implementation and tests were prepared with Codex.

Test instructions

• yarn test:unit --spec packages/browser-rum-core/src/domain/tracing/tracer.spec.ts — 43 tests passed.
• ESLint passed on the three changed files.
• Prettier passed on the three changed files.
• TypeScript passed for the changed tracing files and the E2E project.
• The focused E2E run could not start locally because the Next.js test-app dependency is not installed (next: command not found).

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

[datadog-datadog-prod-us1]

✅ All CI checks and tests passed.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 77.02% (+0.01%)

_{This comment will be updated automatically if new data arrives.
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[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

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