feat(browser): Improve idle span handling

[mydea]

Closes #12051

This PR does two things:

1. Ensures we add helpful attributes to idle spans more consistently. We now add the sentry.idle_span_finish_reason attribute always for idle spans, not only when op == ui.action.click. This should also make it easier to debug stuff etc, and I see no reason to not always set this. Additionally, we also keep the number of spans we discarded (if any) for easier debugging too (as sentry.idle_span_discarded_spans).
2. We ensure that idle spans cannot exceed the configured finalTimeout. Previously, due to the order of things, it was possible that we ended a span very late, if it had a child span with a very late end timestamp (as we took the last child span timestamp). Possibly this could lead to overly long transactions.

I think 2, combined with the fact that later we do filter out child spans that ended after idle span, lead to the incorrect-length spans.

[Lms24]

Good catches!

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser	21.67 KB (0%)
@sentry/browser (incl. Tracing)	32.75 KB (+0.09% 🔺)
@sentry/browser (incl. Tracing, Replay)	68.09 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	61.49 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas)	72.13 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback)	84.14 KB (+0.05% 🔺)
@sentry/browser (incl. Feedback)	37.6 KB (0%)
@sentry/browser (incl. sendFeedback)	26.23 KB (0%)
@sentry/browser (incl. FeedbackAsync)	30.6 KB (0%)
@sentry/react	24.36 KB (0%)
@sentry/react (incl. Tracing)	35.71 KB (+0.08% 🔺)
@sentry/vue	25.5 KB (0%)
@sentry/vue (incl. Tracing)	34.54 KB (+0.09% 🔺)
@sentry/svelte	21.8 KB (0%)
CDN Bundle	24.21 KB (0%)
CDN Bundle (incl. Tracing)	34.16 KB (+0.08% 🔺)
CDN Bundle (incl. Tracing, Replay)	67.84 KB (+0.04% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback)	72.8 KB (+0.04% 🔺)
CDN Bundle - uncompressed	71.21 KB (0%)
CDN Bundle (incl. Tracing) - uncompressed	101.31 KB (+0.07% 🔺)
CDN Bundle (incl. Tracing, Replay) - uncompressed	210.93 KB (+0.04% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	223.23 KB (+0.04% 🔺)
@sentry/nextjs (client)	34.95 KB (+0.09% 🔺)
@sentry/sveltekit (client)	33.31 KB (+0.1% 🔺)
@sentry/node	140.99 KB (0%)
@sentry/aws-serverless	128.03 KB (0%)