fix(hogql): fix trends actors with monthly active users aggregation

[thmsobrmlr]

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Problem

https://posthoghelp.zendesk.com/agent/tickets/12749 trends persons modal (cannot redefine actor_id)

It's not possible to get the actors of a trends query with monthly active users series (and potentially other aggregations).

Changes

Todos

• [x] Add test
• [ ] Fix new error "Unable to resolve field: timestamp"

How did you test this code?

[sentry[bot]]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: posthog/hogql_queries/insights/trends/trends_query_builder.py

Function	Unhandled Issue
_get_events_subquery	ValueError: could not convert string to float: '[2876.0,3177.01]' posthog.tasks.tasks.p... Event Count: 1

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[thmsobrmlr]

Closing in favor of https://github.com/PostHog/posthog/pull/22132