Workfront Planning V2 POST /v2/record-types/{rtId}/fields returns null-Field lookup error for reference + formula creates

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What feature should be added?

Affected surface:
- POST /v2/record-types/{rtId}/fields with type: "reference"
- POST /v2/record-types/{rtId}/fields with type: "formula"

API version: Planning API V2 (May 2026 GA)

Symptoms:
For type: "reference", the documented body shape (exactly per developer.adobe.com/wf-planning/api/v2):
```
{
"displayName": "Campaign Capacity",
"type": "reference",
"referenceOptions": { "multiple": true, "recordTypeId": "Rt6a..." }
}
```
returns HTTP 404 with:
```
{
"title": "Field with id null is not found",
"errorCode": "ENTITY_WITH_ATTRIBUTE_NOT_FOUND",
"messageArguments": { "entity": "Field", "attribute": "id", "value": null }
}
```

The handler is performing an internal Field.findById(null) lookup that is never exposed to or controllable by the caller.

For type: "formula" with any documented expression, the endpoint hangs ~75 seconds then returns HTTP 500 INTERNAL_SERVER_ERROR.

Sample requestIds (from today's traffic):

- Reference field requestIds: c6013b15-aae6-499c-8702-a2124877d028, f2e87015-4718-4da9-924d-10229294acb6, daa5ebd0-de8c-4397-bd2d-6417eb443c2b, 102d7507-fc32-4196-9eca-52c3c78fb4af, bab6ce93-18b9-4e4b-8107-3557986c34d0, ea25a376-2d3c-4381-b17f-5666c900b8f7
- Formula field requestIds: 7e4f0768-e9c2-44fe-b961-e6f5889bc6d0, 105bb253-f10d-44ba-96c5-71b15046a249, ddc01350-35a7-45c7-a512-83eeda67f1b5, 8c7a5aed-e8ee-4d00-ac6b-0439bb960e4e

Reproducibility: 100%. Every reference + formula create against this instance fails identically. Scalar field creates on the same endpoint (text, single-select, date, number, etc.) succeed normally - only reference + formula are broken.

Impact: Blocks any cross-workspace migration or backup/restore workflow that needs to recreate schema. Currently working around by importing scalar fields + records and re-creating reference + formula fields by hand in the WP UI.

### What is the expected behavior?

Expected: HTTP 201 with the created field, per V2 spec.

### How will this feature improve the user experience?

API

### What would a solution for this issue look like?

_No response_

Start by reproducing the documented reference and formula POST requests and compare them with the scalar-field creates that succeed. Trace the implementation behind POST /v2/record-types/{rtId}/fields and add or update coverage so both field types return HTTP 201 without the null-Field error or timeout.

Written by the indexing model from the issue text.