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### What?
This PR fixes an issue where the bottom "Create new ..." button would
cause a runtime error due to not accounting for a polymorphic join
setup.
### Why?
To prevent a runtime error and allow users the ability to add new
documents to the join as expected even in a polymorphic setup.
### How?
Creation of a new `AddNewButton` which handles all of the add new button
instances in the `RelationshipTable` component.
Addresses
https://github.com/payloadcms/payload/issues/12913#issuecomment-3001475438
Before:
[join-polymorphic-runtime-error--Payload.webm](https://github.com/user-attachments/assets/fad3a1ba-c51c-4731-84cc-c27adbaac1d9)
After:
[polymorphic-after-Editing---Multiple-Collections-Parent---Payload
(1).webm](https://github.com/user-attachments/assets/e3baf902-1b2b-4f19-8b6d-838edd6fef80)
a1822d21d0