fix(browser): Correctly parse sampleRate when consistentTraceSampling is enabled

[Lms24]

This PR fixes 2 bugs in our trace linking logic that would emerge when consistentTraceSampling was enabled:

1. We'd incorrectly parse an undefined sample rate as 0 on the last DSC instead of falling back to the sampling rate on the span
2. The order of checking for the sample rate on the span vs. DSC was incorrect. This just didn't surface because of bug 1

closes#21264

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 87d27eb. Configure here.}

[github-actions]

size-limit report 📦

⚠️Warning: Base artifact is not the latest one, because the latest workflow run is not done yet. This may lead to incorrect results. Try to re-run all tests to get up to date results.

Path	Size	% Change	Change
@sentry/browser	27.2 kB	-	-
@sentry/browser - with treeshaking flags	25.65 kB	-	-
@sentry/browser (incl. Tracing)	45.25 kB	+0.02%	+8 B 🔺
@sentry/browser (incl. Tracing + Span Streaming)	47.48 kB	+0.02%	+7 B 🔺
@sentry/browser (incl. Tracing, Profiling)	50.24 kB	+0.01%	+3 B 🔺
@sentry/browser (incl. Tracing, Replay)	84.84 kB	+0.02%	+10 B 🔺
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	74.41 kB	+0.02%	+9 B 🔺
@sentry/browser (incl. Tracing, Replay with Canvas)	89.55 kB	+0.02%	+10 B 🔺
@sentry/browser (incl. Tracing, Replay, Feedback)	102.17 kB	+0.01%	+10 B 🔺
@sentry/browser (incl. Feedback)	44.36 kB	-	-
@sentry/browser (incl. sendFeedback)	32.02 kB	-	-
@sentry/browser (incl. FeedbackAsync)	37.11 kB	-	-
@sentry/browser (incl. Metrics)	28.27 kB	-	-
@sentry/browser (incl. Logs)	28.51 kB	-	-
@sentry/browser (incl. Metrics & Logs)	29.22 kB	-	-
@sentry/react	29.03 kB	-	-
@sentry/react (incl. Tracing)	47.53 kB	+0.01%	+3 B 🔺
@sentry/vue	32.22 kB	-	-
@sentry/vue (incl. Tracing)	47.17 kB	+0.02%	+5 B 🔺
@sentry/svelte	27.23 kB	-	-
CDN Bundle	29.57 kB	-	-
CDN Bundle (incl. Tracing)	47.82 kB	+0.03%	+11 B 🔺
CDN Bundle (incl. Logs, Metrics)	31.08 kB	-	-
CDN Bundle (incl. Tracing, Logs, Metrics)	49.06 kB	+0.02%	+9 B 🔺
CDN Bundle (incl. Replay, Logs, Metrics)	70.34 kB	-	-
CDN Bundle (incl. Tracing, Replay)	85.19 kB	+0.01%	+8 B 🔺
CDN Bundle (incl. Tracing, Replay, Logs, Metrics)	86.35 kB	+0.01%	+7 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback)	91.06 kB	+0.01%	+8 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback, Logs, Metrics)	92.2 kB	+0.01%	+7 B 🔺
CDN Bundle - uncompressed	87.7 kB	-	-
CDN Bundle (incl. Tracing) - uncompressed	144.13 kB	+0.02%	+20 B 🔺
CDN Bundle (incl. Logs, Metrics) - uncompressed	92.19 kB	-	-
CDN Bundle (incl. Tracing, Logs, Metrics) - uncompressed	147.88 kB	+0.02%	+20 B 🔺
CDN Bundle (incl. Replay, Logs, Metrics) - uncompressed	216.91 kB	-	-
CDN Bundle (incl. Tracing, Replay) - uncompressed	262.9 kB	+0.01%	+20 B 🔺
CDN Bundle (incl. Tracing, Replay, Logs, Metrics) - uncompressed	266.65 kB	+0.01%	+20 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	276.6 kB	+0.01%	+20 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback, Logs, Metrics) - uncompressed	280.34 kB	+0.01%	+20 B 🔺
@sentry/nextjs (client)	50.05 kB	+0.02%	+9 B 🔺
@sentry/sveltekit (client)	45.71 kB	+0.02%	+7 B 🔺
@sentry/core/server	75.94 kB	-	-
@sentry/core/browser	63.09 kB	-	-
@sentry/node-core	61.69 kB	-0.01%	-1 B 🔽
@sentry/node	130.4 kB	-	-
@sentry/node - without tracing	74.06 kB	+0.01%	+1 B 🔺
@sentry/aws-serverless	86.26 kB	-	-
@sentry/cloudflare (withSentry) - minified	171.49 kB	-	-
@sentry/cloudflare (withSentry)	428.63 kB	-	-

View base workflow run

[JPeer264]

LGTM. Nice catch!