✨ Add CDN default headers

[bdibon]

Motivation

The RUM backend's shared-cache detection reads the x-cache, cf-cache-status, and x-vercel-cache response headers to flag cache_reason=shared_cache_hit. Only x-cache was collected by default, so Cloudflare and Vercel CDN traffic was invisible to that detection.

Changes

• Added cf-cache-status and x-vercel-cache to DEFAULT_TRACKED_RESOURCE_HEADERS
• Customers using trackResourceHeaders: true will now automatically collect these headers, no config change required on their end.

Test instructions

• yarn test:unit --spec packages/browser-rum-core/src/domain/configuration/configuration.spec.ts — existing test asserting trackResourceHeaders: true resolves to DEFAULT_TRACKED_RESOURCE_HEADERS.map((name) => ({ name })) covers the updated list.

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

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	178.25 KiB	178.29 KiB	+35 B	+0.02%	✅
Rum Profiler	8.22 KiB	8.22 KiB	0 B	0.00%	✅
Rum Recorder	21.14 KiB	21.14 KiB	0 B	0.00%	✅
Logs	55.47 KiB	55.47 KiB	0 B	0.00%	✅
Rum Slim	136.17 KiB	136.20 KiB	+35 B	+0.03%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅

[datadog-datadog-us1-prod]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 77.26% (+0.00%)

_{This comment will be updated automatically if new data arrives.
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