testStandbyActor cannot test multi-tenant standby actors

[gytelio]

testStandbyActor is only able to test single tenant standby mode.

Multi tenant actors cannot be used through tasks

Suggested fix

Add a mode that skips the Task and calls the Actor's own standby URL directly?

Two teams have hit this:

• apify-store/facebook PR #1306 (pages scraper standby)
• apify-store/google-maps PR #1990 (reviews scraper standby)

[gytelio]

How should the build be handled, because you cannot pin to a task, so the only lever how to pin is manually add an exact build number to a production actor?

[gytelio]

So we can start implementing this after https://apify.slack.com/archives/C059M9L8ZLP/p1788428851113129?thread_ts=1788189777.679699&cid=C059M9L8ZLP