✨ Add webSocketObservable for WebSocket lifecycle events

[bdibon]

Motivation

Provide a low-level observable that emits WebSocket lifecycle events (connecting, open, message in/out, close). This is the browser-level building block consumed by the WebSocket collection later in the stack.

Changes

• Add webSocketObservable (initWebSocketObservable), instrumenting the WebSocket constructor and send, and listening to open / message / close / error events.
• Add WebSocketEventMap support to addEventListener.
• Export the observable and its context types from @datadog/browser-core.

Test instructions

yarn test:unit --spec packages/core/src/browser/webSocketObservable.spec.ts

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

[bdibon]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• ✨ Collect WebSocket resource events #4718
• ✨ Add trackWebSockets configuration option (default: false) #4717
• 🔧 Sync rum-events-format with WebSocket resource schema #4716
• ✨ Add webSocketObservable for WebSocket lifecycle events #4715 👈 (View in Graphite)
• ⚗️ Add instrumentConstructor utility #4714
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	171.88 KiB	171.88 KiB	0 B	0.00%	✅
Rum Profiler	7.88 KiB	7.88 KiB	0 B	0.00%	✅
Rum Recorder	21.22 KiB	21.22 KiB	0 B	0.00%	✅
Logs	54.39 KiB	54.39 KiB	0 B	0.00%	✅
Rum Slim	129.77 KiB	129.77 KiB	0 B	0.00%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅

[datadog-datadog-prod-us1]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 88.10%
• Overall Coverage: 76.78% (+0.04%)

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