insights: add export handler back with bugfix for search context loader

[leonore]

revert revert for #46662 and add fix

Test plan

See original PR for full test plan

Observed panic when using a search context with insight on main.
No longer saw panic either on insights page or using the export endpoint after fix

[sourcegraph-bot]

Codenotify: Notifying subscribers in CODENOTIFY files for diff 47025db...48bf3c2.

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File(s)

@sourcegraph/code-insights-backend

enterprise/internal/insights/httpapi/export.go enterprise/internal/insights/insights.go enterprise/internal/insights/resolvers/insight_series_resolver.go enterprise/internal/insights/resolvers/insight_view_resolvers_test.go enterprise/internal/insights/store/insight_store.go enterprise/internal/insights/store/mocks_temp.go enterprise/internal/insights/store/mocks_test.go enterprise/internal/insights/store/permissions.go enterprise/internal/insights/store/search_contexts.go enterprise/internal/insights/store/store.go enterprise/internal/insights/store/store_test.go