⚗️ [RUM-18031] manual resources participate in pending-activity tracking (prototype)

[bcaudan]

Motivation

Prototype for the electron-sdk IPC RUM events work: manual resources (started/stopped via the public manual resource API) did not hold page activity "busy" the way real network requests do, so IPC-driven activity could be missed by pending-activity tracking (e.g. view loading time, action duration).

Changes

• trackManualResources now notifies REQUEST_STARTED / REQUEST_COMPLETED life-cycle events when a manual resource starts/stops, so it participates in pending-activity tracking like fetch/XHR requests.
• Added getNextRequestIndex() export in requestCollection to share request indexing with manual resources.
• Added RequestType.OTHER to resourceUtils for manual resources reported through this path.
• This is unconditional for all callers of the manual resource API — noted as a prototype behavior to revisit with the browser-sdk team before shipping outside this experiment.

Test instructions

1. Run unit tests: yarn test:unit --spec packages/browser-rum-core/src/domain/resource/trackManualResources.spec.ts
2. Verify the new "pending activity tracking" spec cases pass, confirming REQUEST_STARTED/REQUEST_COMPLETED are fired and page activity is held busy while a manual resource is in flight.

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 green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 77.21% (-0.00%)

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[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	180.44 KiB	180.67 KiB	+236 B	+0.13%	✅
Rum Profiler	8.43 KiB	8.43 KiB	0 B	0.00%	✅
Rum Recorder	21.12 KiB	21.12 KiB	0 B	0.00%	✅
Logs	57.04 KiB	57.04 KiB	0 B	0.00%	✅
Rum Salesforce	N/A	138.71 KiB	N/A	N/A	N/A
Rum Slim	138.47 KiB	138.71 KiB	+242 B	+0.17%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅
Rum Shopify	N/A	200.25 KiB	N/A	N/A	N/A
Rum-shopify Profiler	N/A	8.43 KiB	N/A	N/A	N/A
Rum-shopify Recorder	N/A	3.72 KiB	N/A	N/A	N/A