fix(core): Instrument invoke_agent root span, and support Vercel ai v5

[RulaKhaled]

Main changes are:

1. Instrument root invoke_agent span: This addresses that transaction event.spans only includes child spans, ensuring proper correlation beyond just children.
2. Refactor vercelai integration layout in core: Restructured the vercelai folder to mirror our other AI agent integrations (split constants/utils/index) for clarity and easier maintenance.
3. Link errors in Vercel AI v5 (metadata-based): Added support to capture and link tool errors that are returned as result metadata (not thrown). We correlate tool-call IDs to spans and set the trace/span context when capturing the error, so errors are attached to the right tool-call/trace even without a thrown exception.
4. Added tests to test vercel ai v5

[github-actions]

size-limit report 📦

Path	Size	% Change	Change
@sentry/browser	24.16 kB	-	-
@sentry/browser - with treeshaking flags	22.73 kB	-	-
@sentry/browser (incl. Tracing)	39.87 kB	-	-
@sentry/browser (incl. Tracing, Replay)	77.99 kB	-	-
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	67.83 kB	-	-
@sentry/browser (incl. Tracing, Replay with Canvas)	82.69 kB	-	-
@sentry/browser (incl. Tracing, Replay, Feedback)	94.79 kB	-	-
@sentry/browser (incl. Feedback)	40.83 kB	-	-
@sentry/browser (incl. sendFeedback)	28.81 kB	-	-
@sentry/browser (incl. FeedbackAsync)	33.7 kB	-	-
@sentry/react	25.88 kB	-	-
@sentry/react (incl. Tracing)	41.86 kB	-	-
@sentry/vue	28.64 kB	-	-
@sentry/vue (incl. Tracing)	41.69 kB	-	-
@sentry/svelte	24.18 kB	-	-
CDN Bundle	25.66 kB	-	-
CDN Bundle (incl. Tracing)	39.76 kB	-	-
CDN Bundle (incl. Tracing, Replay)	75.82 kB	-	-
CDN Bundle (incl. Tracing, Replay, Feedback)	81.27 kB	-	-
CDN Bundle - uncompressed	74.96 kB	-	-
CDN Bundle (incl. Tracing) - uncompressed	117.59 kB	-	-
CDN Bundle (incl. Tracing, Replay) - uncompressed	231.88 kB	-	-
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	244.48 kB	-	-
@sentry/nextjs (client)	43.89 kB	-	-
@sentry/sveltekit (client)	40.32 kB	-	-
@sentry/node-core	47.99 kB	-	-
@sentry/node	147.17 kB	+0.11%	+152 B 🔺
@sentry/node - without tracing	92.24 kB	-	-
@sentry/aws-serverless	103.67 kB	+0.01%	+1 B 🔺

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