✨ implement remote configuration async loading & caching

[gogusarov]

Motivation

Support remote configuration asynchronous loading & caching mechanism according to the RFC

Changes

Added async remote configuration fetching flow where config isn't awaited during DD_RUM.init but read synchronously from a versioned localStorage cache (dd_rc_<remoteConfigurationId>) and applied on the same page load. A background fetch runs in parallel and refreshes the cache for the next page load, so updates roll out one load late but startup is unblocked. Cache outcomes (hit/miss/error) and fetch outcomes (success/failure) are reported via the existing telemetry metrics. Unit tests cover the new cache module and the read/apply/fetch flow; the e2e scenarios were rewritten to wait for the cache to be populated and reload the page before asserting.

Blocking fetch flow was kept for backward compatibility with existing customer setup

New init configuration API

• remoteConfiguration: { id: string; sync?: boolean } — new top-level field.
  Default behavior is async (cache-and-reload). Pass sync: true to opt into the
  legacy blocking fetch through the new API.
• remoteConfigurationId: string — legacy field preserved for backward
  compatibility with existing customer setups. Always triggers the blocking fetch
  flow.

Test instructions

1. add remote-config.json file to sandbox directory
2. update parameters, passed to DD_RUM.init adding remoteConfiguration: { id: 'remote-config-test' }, remoteConfigurationProxy: '/remote-config.json'
3. check that remote config from file is requested and written to localStorage on the first page load (localStorage.getItem('dd_rc_remote-config-test')
4. check that DD_RUM.getInitConfiguration() returns initial configuration on the first page load
5. check that DD_RUM.getInitConfiguration() returns remote configuration on the second page load

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-prod-us1-6]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 79.63%
• Overall Coverage: 76.97% (+0.01%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: df72f91 | Docs | Datadog PR Page | Give us feedback!}

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	169.51 KiB	170.76 KiB	+1.25 KiB	+0.74%	✅
Rum Profiler	N/A	5.97 KiB	N/A	N/A	N/A
Rum Recorder	21.23 KiB	21.23 KiB	-1 B	-0.00%	✅
Logs	54.70 KiB	54.70 KiB	0 B	0.00%	✅
Rum Slim	N/A	129.08 KiB	N/A	N/A	N/A
Worker	22.99 KiB	22.99 KiB	0 B	0.00%	✅

🚀 CPU Performance

Action Name	Base CPU Time (ms)	Local CPU Time (ms)	𝚫%
addglobalcontext	N/A	0.0033	N/A
RUM - add action	0.0104	0.017	+63.46%
RUM - add error	0.0126	0.016	+26.98%
addtiming	N/A	0.0009	N/A
RUM - start view	0.0109	0.0154	+41.28%
RUM - start/stop session replay recording	0.0008	0.0013	+62.50%
Logs - log message	0.0176	0.0257	+46.02%

🧠 Memory Performance

Action Name	Base Memory Consumption	Local Memory Consumption	𝚫
RUM - add global context	42.69 KiB	38.29 KiB	-4.40 KiB
RUM - add action	67.95 KiB	63.47 KiB	-4.48 KiB
RUM - add timing	37.49 KiB	38.42 KiB	+953 B
RUM - add error	73.15 KiB	70.27 KiB	-2.88 KiB
RUM - start/stop session replay recording	41.06 KiB	41.81 KiB	+768 B
RUM - start view	492.59 KiB	483.80 KiB	-8.79 KiB
Logs - log message	55.59 KiB	55.34 KiB	-256 B

🔗 RealWorld

[thomas-lebeau]

can you edit the PR title to follow our convention please!