fix: check idp existence by org id

[eliobischof]

When creating a new custom policy, the list of IDPs is checked for existence.
For org IDPs, the events are queried by resource owner, which is not necessarily the org ID.
An existing org IDP is not found, and the create custom policy request fails.

This PR explicitly queries by org ID.
It also reuses the central ExistsIDP function to guarantee the same existence checking logic as in other API implementations.

I found this bug when I implemented #7663

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Name	Status	Preview	Comments	Updated (UTC)
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