feat(resource): add read_search.id_attribute for envelope-wrapped APIs

[mbrownnycnyc]

What

Add an optional id_attribute to the read_search block of restapi_object, overriding the
object-wide id_attribute when extracting the matched record's id from a search-result item.

Why

Some APIs wrap the object differently on create vs. in the list/collection endpoint:

• POST /widgets -> {"data": {"id": 123, "name": "foo"}} - the id is at data/id, so you set the
  object-wide id_attribute = "data/id".
• GET /widgets (the read_search source) -> {"data": [{"id": 123, "name": "foo"}]} - the list
  items are flat, with the id at id.

A single object-wide id_attribute can't satisfy both. With id_attribute = "data/id", read_search
cannot find the id in the flat list item, so FindObject errors, the read returns an empty body, and
the resource fails on refresh with:

Error: Error Parsing Plan Data
Could not parse plan data JSON: unexpected end of JSON input

This affects any API that wraps all responses in an envelope (e.g. {code,status,data:{...}} on
create and {code,status,data:[...]} on list).

What changed

• read_search gains an optional id_attribute. When set, FindObject reads the matched item's id
  from that key; otherwise it falls back to the object-wide id_attribute. No behavior change for
  existing configs - fully backward compatible.
• internal/provider/resource_api_object.go (schema + model + map wiring),
  internal/apiclient/object.go (FindObject), and a table test in
  internal/apiclient/object_read_search_test.go.

Example

resource "restapi_object" "widget" {
  path         = "/widgets"   # list endpoint (read_search GETs this)
  create_path  = "/widgets"   # POST returns {"data":{"id":N}}
  id_attribute = "data/id"    # id of the created (wrapped) object

  read_search = {
    search_key   = "name"
    search_value = "foo"
    results_key  = "data"
    id_attribute = "id"       # flat list item: id is at the top level
  }

  data = jsonencode({ name = "foo" })
}

Notes

• Docs under docs/ are generated via tfplugindocs; I was not able to run make docs in my
  environment - happy to regenerate, or a maintainer can.
• Test added; please run CI.