fix(browser): Respect manually set sentry tracing headers in XHR requests

[Lms24]

Analogously to #16183 for fetch, this PR ensures that manually set sentry-trace and baggage headers in XHR requests are respected by our XHR instrumentation and not overwritten.

This also fixes a bug where we'd append multiple sentry-trace or baggage values in case the header was somehow set more than once (analogously to #13907 for fetch)

Added two integration tests to demonstrate correct behaviour.

Came up in https://linear.app/getsentry/issue/FE-380/debug-trace-context-propagation-issue-from-tanstack-to-go-backend

bundle-size wise, this probably amortizes the savings from #16183 🙃

[github-actions]

size-limit report 📦

Path	Size	% Change	Change
@sentry/browser	23.32 KB	-	-
@sentry/browser - with treeshaking flags	23.13 KB	-	-
@sentry/browser (incl. Tracing)	37.01 KB	+0.09%	+32 B 🔺
@sentry/browser (incl. Tracing, Replay)	74.19 KB	+0.06%	+41 B 🔺
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	67.53 KB	+0.06%	+37 B 🔺
@sentry/browser (incl. Tracing, Replay with Canvas)	78.84 KB	+0.06%	+42 B 🔺
@sentry/browser (incl. Tracing, Replay, Feedback)	90.63 KB	+0.05%	+40 B 🔺
@sentry/browser (incl. Feedback)	39.71 KB	-	-
@sentry/browser (incl. sendFeedback)	27.94 KB	-	-
@sentry/browser (incl. FeedbackAsync)	32.7 KB	-	-
@sentry/react	25.12 KB	-	-
@sentry/react (incl. Tracing)	38.99 KB	+0.11%	+40 B 🔺
@sentry/vue	27.55 KB	-	-
@sentry/vue (incl. Tracing)	38.77 KB	+0.1%	+38 B 🔺
@sentry/svelte	23.34 KB	-	-
CDN Bundle	24.52 KB	-	-
CDN Bundle (incl. Tracing)	37.03 KB	+0.11%	+40 B 🔺
CDN Bundle (incl. Tracing, Replay)	72.07 KB	+0.06%	+40 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback)	77.39 KB	+0.06%	+45 B 🔺
CDN Bundle - uncompressed	71.57 KB	-	-
CDN Bundle (incl. Tracing) - uncompressed	109.55 KB	+0.14%	+151 B 🔺
CDN Bundle (incl. Tracing, Replay) - uncompressed	220.84 KB	+0.07%	+151 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	233.36 KB	+0.07%	+151 B 🔺
@sentry/nextjs (client)	40.59 KB	+0.09%	+35 B 🔺
@sentry/sveltekit (client)	37.48 KB	+0.1%	+37 B 🔺
@sentry/node	143.45 KB	-0.01%	-1 B 🔽
@sentry/node - without tracing	96.54 KB	-	-
@sentry/aws-serverless	120.86 KB	-	-

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