feat: Add validate_path for pre-flight API validation during terraform plan

[johnsolomonj]

Summary

Adds two new optional restapi_object resource attributes:

• validate_path — an endpoint to call during terraform plan with the object's data. A non-2xx response immediately fails the plan with the API's error message, surfacing configuration problems before apply.
• validate_method — HTTP method for the validate call (default: POST).

Closes #362.

Motivation

Many REST APIs expose a dedicated validation endpoint (e.g. POST /resource/validate) that performs a dry-run check without persisting anything. Without this feature, validation only surfaces at apply time when the API rejects the POST/PUT, which can cause partial applies where unrelated resources succeed but the invalid one fails.

Usage

resource "restapi_object" "example" {
  path            = "/tables"
  data            = jsonencode({ ... })
  validate_path   = "/tables/validate"
  validate_method = "POST"   # optional, defaults to POST
}

Behaviour

• When validate_path is not set, the feature is a no-op — fully backward compatible.
• When data is unknown at plan time (e.g. depends on another resource), the validate call is skipped to avoid false failures.
• A non-2xx response produces a clear API pre-flight validation failed diagnostic that includes the validate path and the API's error body.

Changes

File	Change
internal/apiclient/object.go	Added ValidatePath/ValidateMethod to APIObjectOpts and APIObject; added ValidateObject() method; added GetValidatePath()/GetValidateMethod() accessors
internal/provider/resource_api_object.go	Added validate_path/validate_method schema attributes and model fields; implemented resource.ResourceWithConfigValidators via validatePathValidator
internal/apiclient/object_validate_test.go	6 unit tests covering no-op, success, custom method, 4xx/5xx rejection, and accessors
internal/provider/resource_api_object_validate_test.go	3 acceptance tests: success (validate called ≥1×), failure (plan error), no validate_path (no-op)

Test results

ok  github.com/Mastercard/terraform-provider-restapi/internal/apiclient
ok  github.com/Mastercard/terraform-provider-restapi/internal/provider

All existing tests pass. go vet and gofmt clean.

[johnsolomonj]

Hi @DRuggeri — would love your review on this when you get a chance! Happy to make any changes needed.