Add `validateForm` mutation

[justlevine]

What problem does this address?

This feature was added to GF 2.6.4, and can serve to provide background async validation to cut down on submission times for large fields.

https://docs.gravityforms.com/validating-forms-with-the-gfapi/

What is your proposed solution?

..

What alternatives have you considered?

No response

Additional Context

No response

[justlevine]

🤖 Automated Triage

Classification: type: feature 🦋 · priority: medium
Agent-feasible: Yes — wraps GFAPI::validate_form_strings() (GF 2.6.4+) and the plugin already has mutation infrastructure to mirror.
Estimated effort: Agent medium · Human medium

Summary: Request to expose GF background async form validation as a GraphQL mutation, mirroring GFAPI::validate_form(). The endpoint exists upstream and the plugin has established mutation patterns to follow, making this a bounded implementation task. Main risk is mapping GF field validation errors onto the GraphQL error format.

Next steps: Ready for planning loop. Recommend modeling the mutation input/output after the existing submitForm/updateDraftEntry mutations and reusing the field value parsing logic.