fix(analytics-browser): diagnostics track user agent (#1428)

[Mercy811]

Summary

Cherry pick #1428

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?:

[Mercy811]

Integration tests failed probably because some mocks in tests are not picked up in this feature branch. Hold this PR until they are solved

[junjie-amplitude]

Can we keep this change in the original branch feat-zoning-1211-plus-diagnostics-user-agent and just pre-release that branch instead? Alvin may need to continue to work on feat-zonging-1211 when he's back

[macroscopeapp]

Track browser user agent in diagnostics by setting AmplitudeBrowser.init to tag user_agent, and set a 10000 ms timeout for one integration Jest test in index.test.ts

Add a user_agent tag using navigator.userAgent to the diagnostics client in AmplitudeBrowser.init in browser-client.ts. Update one integration test to use a 10000 ms Jest timeout in index.test.ts.

📍Where to Start

Start with AmplitudeBrowser.init in browser-client.ts to review how the diagnostics client sets the user_agent tag.

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Macroscope summarized d9c897d.

[macroscopeapp]

Accessing navigator.userAgent without a guard can crash in non‑browser contexts. Consider guarding with typeof navigator !== 'undefined' (like in Context) before setting the tag.

Suggested change

	diagnosticsClient.setTag('user_agent', navigator.userAgent);
	if (typeof navigator !== 'undefined') {
	diagnosticsClient.setTag('user_agent', navigator.userAgent);
	}

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