fix(zendesk): warn on custom-field name collisions

[christophebrun-forest]

Summary

• Zendesk custom fields whose key collides with a native column or relation (e.g. a user_field named role) used to crash agent setup with Field role already defined in collection (RuntimeError).
• The conflicting field is now skipped with a warning via ForestAdminDatasourceZendesk.logger, so agent setup keeps going.
• The filtered list flows back into the datasource search mapping so schema and query translation stay in sync.

Test plan

• bundle exec rspec in packages/forest_admin_datasource_zendesk — 238 examples, 0 failures.
• bundle exec rubocop lib/ spec/ — clean.
• New unit tests in user_spec.rb cover the role collision (warning emitted, native enum schema preserved, native value not overwritten by serializer).

🤖 Generated with Claude Code

Note

Warn and skip custom fields that collide with native field names in Zendesk collections

• Adds add_custom_fields to BaseCollection that checks each custom field against the existing schema, logs a warning on collision, and skips the conflicting field.
• Updates Ticket, User, and Organization collections to use add_custom_fields instead of registering custom fields directly in define_schema, so @custom_fields only contains fields actually added.
• Updates Datasource#build_custom_field_mapping to derive the mapping from the filtered @custom_fields on each collection rather than raw introspection arrays.
• Behavioral Change: native fields (e.g. role on User) can no longer be silently overwritten by a colliding custom field during serialization.

^{Macroscope summarized 3810cf0.}

[PMerlet]

LGTM

[forest-bot]

🎉 This PR is included in version 1.30.4 🎉

The release is available on GitHub release

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