Feature request: Support pre-flight validation via configurable validate endpoint during terraform plan

[johnsolomonj]

Summary

Many REST APIs expose a dedicated validation endpoint (e.g. POST /resource/validate) that performs a dry-run check of a resource configuration without persisting anything. Today there is no way to call this during terraform plan — validation only surfaces at apply time when the API rejects the request.

Proposed solution

Add optional validate_path and validate_method attributes to the restapi_object resource. When set, the provider would call this endpoint during plan (via the ConfigValidators hook in the Terraform Plugin Framework) and fail the plan if the API returns a non-2xx response.

Example configuration:

resource "restapi_object" "example" {
  path            = "/resources"
  data            = jsonencode({ ... })
  validate_path   = "/resources/validate"
  validate_method = "POST"
}

Implementation notes

The Terraform Plugin Framework already supports this via the resource.ResourceWithConfigValidators interface. The work would involve:

1. Adding validate_path / validate_method schema attributes to resource_api_object.go
2. Implementing ConfigValidators() returning a custom validator
3. Adding a ValidateObject() method to APIObject in apiclient/object.go mirroring the existing CreateObject() pattern

Happy to contribute a PR if there is interest.

Why this matters

Catching misconfigured resources at plan time rather than apply time:

• Prevents partial applies where unrelated resources succeed but the invalid one fails
• Gives faster feedback in CI pipelines
• Is especially valuable for complex resource configs where the API has richer validation logic than what can be expressed in Terraform schema