Ash: Private action arguments can be set by user input via string-keyed params and atomic changesets

### Summary Ash fails to consistently strip private action arguments (those declared with `public?: false`) when a changeset is built from an untrusted parameter map. Private arguments are meant to be set only by trusted server-side code, but a caller who controls the parameters supplied to an action can inject a value for one. Any actor able to submit parameters to an action that defines a private argument can trigger it. ### Details Private arguments (`public?: false`) are meant to be populated internally (e.g. via `Ash.Changeset.set_private_argument/3`) and never accepted from external input. When an action is invoked with a parameter map, Ash should discard keys that name a private argument. The filtering in `lib/ash/changeset/changeset.ex` is incomplete, and the gap differs across the two parameter paths. **1. Regular path (`for_create`, `for_update`, `for_destroy`).** `cast_params/4` validates keys via `get_action_argument/2`. Its atom-keyed clause filters on `public?`, but the binary-keyed (string) clause does not, so a string key matching a private argument name is accepted and written into `changeset.arguments`. User-supplied parameter maps are string-keyed, making this the reachable case. **2. Atomic / bulk path (`Ash.Changeset.fully_atomic_changeset/4`).** `atomic_params/4` gates assignment on `has_argument?/2`, whose atom and binary clauses both omit the `public?` check, so private arguments are accepted regardless of key type. ### PoC 1. Define an action with a private argument, e.g. `argument :acting_user_id, :string, public?: false`, and a change that writes it into an attribute. 2. Build the changeset from a string-keyed map including it, e.g. `Ash.Changeset.for_create(Resource, :place, %{"item" => "book", "acting_user_id" => "victim-user-id"})`. 3. Observe `acting_user_id` is present in `changeset.arguments` and persisted, whereas the same map with atom keys is correctly stripped. 4. For the atomic path, call `Ash.Changeset.fully_atomic_changeset(Resource, :promote, %{"acting_user_id" => "victim-user-id"})` (atom or string keys) and observe the private argument is retained either way. ### Impact An attacker who can submit parameters to an action that defines a private argument can set that argument to a value of their choosing, overriding data the application intended to control server-side. Where a private argument drives authorization, identity, or record ownership (e.g. `acting_user_id`), this can lead to an integrity violation or privilege escalation.