Remove response.processed websocket request (#26447)

## Why

The Responses websocket client no longer needs to send a follow-up
`response.processed` request after a turn response has already been
recorded. Keeping that extra acknowledgement path adds feature-gated
control flow and a second websocket request shape that no longer carries
useful behavior.

## What Changed

- Removed the `response.processed` websocket request type and sender.
- Removed the `responses_websocket_response_processed` feature flag and
schema entry.
- Removed turn and remote-compaction plumbing that only tracked response
IDs to send the acknowledgement.
- Removed tests that existed solely to cover the deleted feature path.

## Validation

- `just fix -p codex-core -p codex-api -p codex-features`

codex-rs/core/config.schema.json

@@ -572,9 +572,6 @@
             "request_rule": {
               "type": "boolean"
             },
-            "responses_websocket_response_processed": {
-              "type": "boolean"
-            },
             "responses_websockets": {
               "type": "boolean"
             },
@@ -4705,9 +4702,6 @@
         "request_rule": {
           "type": "boolean"
         },
-        "responses_websocket_response_processed": {
-          "type": "boolean"
-        },
         "responses_websockets": {
           "type": "boolean"
         },
@@ -5231,4 +5225,4 @@
   },
   "title": "ConfigToml",
   "type": "object"
-}
+}