♻️ move batching transport stack to @datadog/js-core

[BenoitZugmeyer]

Motivation

The @datadog/js-core package is meant to hold runtime-agnostic SDK utilities
shared across all Datadog JS SDKs. However, the entire batching transport stack
(createBatch, createHttpRequest, createFlushController, and all their
dependencies) lived exclusively in @datadog/browser-core, making it
unavailable to non-browser consumers such as the Worker or future server-side
SDK targets.

This PR moves the complete transport stack into @datadog/js-core, with
browser-specific concerns (fetch, sendBeacon, DOM events) remaining in
browser-core as injectable strategies.

Changes

• Pure utilities (byteUtils, jsonStringify, context, objectValues,
  isServerError) moved to @datadog/js-core/util; browser-core re-exports.
• mockable moved to @datadog/js-core/util.
• Observable, BufferedObservable, mergeObservables, queueMicrotask
  moved to @datadog/js-core/util; queueMicrotask drops the monitor()
  wrapper (callers handle error wrapping at their own layer).
• Encoder / createIdentityEncoder moved to @datadog/js-core/transport.
• timer / getZoneJsOriginalValue moved to @datadog/js-core/util;
  js-core timer has no monitor() wrapping; browser-core keeps monitored wrappers.
• sendWithRetryStrategy and Payload types moved to
  @datadog/js-core/transport; navigator.onLine access goes through
  globalObject.
• flushController moved to @datadog/js-core/transport; PageExitReason
  and PageMayExitEvent move with it as pure types; pageMayExitObservable
  is injected as a parameter.
• createHttpRequest moved to @datadog/js-core/transport with injectable
  sendStrategy / sendOnExitStrategy; browser-core keeps fetchStrategy and
  sendBeaconStrategy and wraps the js-core factory.
• createBatch moved to @datadog/js-core/transport; request and
  pageMayExitObservable are injected parameters; display.warn becomes an
  injected warn function; browser-core wrapper preserves the existing call
  signature for all current callers.
• API cleanup: sendWithRetryStrategy, newRetryState, RetryState,
  internal constants, createFlushController, and FlushController are
  removed from js-core's public transport entry — they are internal details.
  Their specs move to js-core alongside the implementation.

Test instructions

This is a pure refactor with no observable behaviour change. All existing unit
tests cover the moved code. To sanity-check the SDK still initialises and
sends data correctly:

1. Run yarn dev and open http://localhost:8080.
2. Check the Network tab — RUM and Logs requests should still be sent to the
   intake endpoints on the configured interval.
3. Navigate away from the page and confirm a flush request is sent on
   beforeunload/visibilitychange.

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-us1-2]

 

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ℹ️ Info

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🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 82.92%
• Overall Coverage: 77.16% (+0.20%)

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Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	181.52 KiB	181.80 KiB	+291 B	+0.16%	✅
Rum Profiler	8.43 KiB	8.43 KiB	-1 B	-0.01%	✅
Rum Recorder	22.31 KiB	22.31 KiB	-2 B	-0.01%	✅
Logs	57.63 KiB	57.97 KiB	+349 B	+0.59%	✅
Rum Salesforce	N/A	139.94 KiB	N/A	N/A	N/A
Rum Slim	139.59 KiB	139.93 KiB	+352 B	+0.25%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅
Rum Shopify	N/A	202.63 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