✅ Fix React router test state leak

[watson]

Motivation

While investigating #4750, the BrowserStack Firefox unit job failed with TypeError: rumPublicApi.startView is not a function in reactPlugin.spec.ts.

CI logs: https://gitlab.ddbuild.io/datadog/browser-sdk/-/jobs/1751673091

The failure was order-dependent: wrapCreateRouter regression tests could register pending React router onRumInit subscribers without clearing the React plugin global state, then a later spec initialized the plugin with a minimal fake public API.

Changes

Reset the React plugin before the wrapCreateRouter — initial error forwarding regression tests and register cleanup afterward, so pending subscribers cannot leak into later randomized specs.

This only affects test isolation.

Test instructions

yarn test:unit --spec packages/browser-rum-react/src/domain/reactRouter/createRouter.spec.ts --spec packages/browser-rum-react/src/domain/reactPlugin.spec.ts --seed 11837

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

[watson]

• ✅ Fix React router test state leak #4751 👈 (View in Graphite)
• main

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[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	171.86 KiB	171.86 KiB	0 B	0.00%	✅
Rum Profiler	7.88 KiB	7.88 KiB	0 B	0.00%	✅
Rum Recorder	21.21 KiB	21.21 KiB	0 B	0.00%	✅
Logs	54.31 KiB	54.31 KiB	0 B	0.00%	✅
Rum Slim	129.69 KiB	129.69 KiB	0 B	0.00%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅

[datadog-prod-us1-5]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 76.77% (+0.00%)

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