fix(issues): Route adjacent-event lookups to Events dataset

[yuvmen]

Fixes SENTRY-5NPK.

Navigating prev/next on an issue events page crashed with QueryIllegalTypeOfArgument: Argument 0 for function arrayConcat must be an array but it has type String whenever the search query filtered an ARRAY_FIELDS column (!error.type:[A,B], stack.function:[X], etc.).

Why It Started 9 Days Ago

#113234 (Apr 17) wired the issue search query into prev/next event navigation, behind a sentry-org allowlist. #113555 (Apr 21 — the day this issue first fired) flipped that to a boolean option. #114063 (Apr 27) made it the default for everyone. Before this rollout, prev/next ignored the query, so the broken SQL was never generated. The dataset asymmetry below was always there but only mattered once filter conditions started flowing through.

Why Switching to Events Is the Right Call (Not as Big as It Reads)

get_adjacent_event_ids was the last event-fetch method on the issue events page still using Dataset.Discover. The methods that load the initial event on the very same page already use Dataset.Events for error groups:

# src/sentry/models/group.py:260 — get_oldest_or_latest_event / get_recommended_event
if group.issue_category == GroupCategory.ERROR:
    dataset = Dataset.Events
else:
    dataset = Dataset.IssuePlatform

So when a user opens an issue, the first event loads through Events, and then prev/next was sending the same group's same conditions to Discover. Both datasets read from the same underlying ClickHouse errors storage; the difference is entity-level column resolution. The Discover entity tag-promotes columns like exception_stacks.type to tags.value[indexOf(tags.key, ...)] (a scalar String), which makes the existing hasAny(arrayConcat(name), array(...)) shape produced by convert_search_filter_to_snuba_query blow up. The Events entity treats those columns as the native nested arrays they are, and the same SQL parses fine.

The original justification for Discover was a comment claiming it was needed "to enable getting events across both errors and transactions, which is required when doing pagination in discover." Adjacent-event lookups always run within a single group, which is bounded to one entity, so the cross-entity Discover view isn't needed here. The stale comment is removed.

The change is scoped: only the non-transaction, non-occurrence branch of _get_dataset_for_event flips. Transactions still go to Transactions, occurrences/generic still go to IssuePlatform. Only error events change, and they now match what the rest of the page already does.

On Test Coverage

No regression test in this PR. Local devservices Snuba's discover entity treats exception_stacks.type as a real nested array column, so the production failure mode (tag-promotion to a scalar) doesn't reproduce here. The existing endpoint tests around prev/next navigation still pass.