Add AppRole response schema validation tests (#18636)

This PR modifies every test in `builtin/credentials/approle/path_role_test.go` with new validation checks to ensure that approle/path_role  successful responses align with the declared response schema.

It also introduces a test helper in `sdk/helper/testhelpers`:

```go
func FindResponseSchema(t *testing.T, ...)
```

This test helper will be useful for all plugins that require similar response schema validation in tests.

### Background

This PR is part of the ongoing work to add structured responses in Vault OpenAPI (VLT-234)