✨ Add browser tab ID to all RUM and Logs events

[mormubis]

Motivation

The Browser SDK shares session state across tabs via cookies/localStorage, but has no way to identify which specific tab produced a given event. Adding _dd.browser_tab_id enables distinguishing events from different tabs within the same session.

Changes

• New tabContext module (packages/core/src/domain/contexts/tabContext.ts): Generates a UUID v4 per browser tab, persisted in sessionStorage so it survives page reloads. Falls back to in-memory UUID when sessionStorage is unavailable (Web Workers, sandboxed iframes).
• Schema update: Added browser_tab_id to _dd properties in _common-schema.json and logsEvent.types.ts.
• Wired into RUM and Logs: Both SDKs register the tab context via the Assemble hook, so every event includes _dd.browser_tab_id.
• Unit tests: 8 tests covering hook output, ID consistency, sessionStorage persistence/reuse, and in-memory fallback.

Behavior

Scenario	Behavior
New tab	Generates UUID, persists to sessionStorage
Page reload	Reuses existing UUID from sessionStorage
Tab duplication	Both tabs share the same ID (accepted tradeoff)
sessionStorage unavailable	In-memory UUID per page load

Test instructions

```bash
yarn test:unit --spec packages/core/src/domain/contexts/tabContext.spec.ts
yarn test:unit # full suite passes
yarn typecheck
yarn lint
```

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

[github-actions]

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I have read the CLA Document and I hereby sign the CLA

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1 out of 2 committers have signed the CLA.
✅ (mormubis)[https://github.com/mormubis]
❌ @claude
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