[RUM-15280] ✨ Expose DEFAULT_TRACKED_RESOURCE_HEADERS on RumPublicApi

[bdibon]

Motivation

Dogfooding feedback on trackResourceHeaders: customers using the CDN bundle have no runtime access to the default header list, so they can't extend it (e.g. [...defaults, 'x-my-header']) without hardcoding. Named exports from entries/main.ts reach NPM users but not the window.DD_RUM global, so the constant needs to be attached to the public API object itself.

Changes

• Expose DEFAULT_TRACKED_RESOURCE_HEADERS as a property on RumPublicApi (marked @hidden for now), so CDN users can access it as window.DD_RUM.DEFAULT_TRACKED_RESOURCE_HEADERS.
• Update the trackResourceHeaders JSDoc example in RumInitConfiguration to use ...DEFAULT_TRACKED_RESOURCE_HEADERS.map((h) => ({ name: h })) instead of the name: (h) => ...includes(h) predicate form — matches how customers actually want to extend the defaults.

Note: this introduces a non-function, non-state property on RumPublicApi for the first time; every other member is either a method, a ContextManager, initConfiguration, or version.

Test instructions

1. Check out the branch and run yarn build (or yarn dev).
2. Load the sandbox page with the CDN bundle in a browser.
3. In DevTools console, run:

   window.DD_RUM.DEFAULT_TRACKED_RESOURCE_HEADERS

   Expect the array ['cache-control', 'etag', 'age', 'expires', 'content-type', 'content-encoding', 'vary', 'content-length', 'server-timing', 'x-cache'].
4. Verify it can be used to extend the config:

   window.DD_RUM.init({
     applicationId: '<APP_ID>',
     clientToken: '<CLIENT_TOKEN>',
     trackResourceHeaders: [
       ...window.DD_RUM.DEFAULT_TRACKED_RESOURCE_HEADERS.map((h) => ({ name: h })),
       { name: 'x-request-id' },
     ],
   })

   and that a resource event carries the expected headers.

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	179.27 KiB	179.27 KiB	+2 B	+0.00%	✅
Rum Profiler	6.16 KiB	6.16 KiB	0 B	0.00%	✅
Rum Recorder	27.03 KiB	27.03 KiB	0 B	0.00%	✅
Logs	56.65 KiB	56.65 KiB	0 B	0.00%	✅
Rum Slim	135.11 KiB	135.14 KiB	+32 B	+0.02%	✅
Worker	23.63 KiB	23.63 KiB	0 B	0.00%	✅

🚀 CPU Performance

Pending...

🧠 Memory Performance

Action Name	Base Memory Consumption	Local Memory Consumption	𝚫
RUM - add global context	32.86 KiB	32.03 KiB	-847 B
RUM - add action	105.90 KiB	100.44 KiB	-5.45 KiB
RUM - add timing	31.38 KiB	32.01 KiB	+645 B
RUM - add error	103.00 KiB	93.54 KiB	-9.45 KiB
RUM - start/stop session replay recording	31.12 KiB	32.44 KiB	+1.32 KiB
RUM - start view	494.41 KiB	487.89 KiB	-6.52 KiB
Logs - log message	95.78 KiB	102.68 KiB	+6.90 KiB

🔗 RealWorld

[datadog-datadog-prod-us1]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 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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