feat: configure acquire chat batch size (#23196)

## Summary
- add a hidden deployment config option for chat acquire batch size
(`CODER_CHAT_ACQUIRE_BATCH_SIZE` / `chat.acquireBatchSize`)
- thread the configured value into chatd startup while preserving the
existing default of `10`
- clamp the deployment value to the `int32` range before passing it into
chatd
- regenerate the API/docs/types/testdata artifacts for the new config
field

## Why
`chatd` currently acquires pending chats in batches of `10` via a
compile-time default. This change makes that batch size
operator-configurable from deployment config, so we can tune acquisition
behavior without another code change.

docs/reference/api/general.md

@@ -204,6 +204,9 @@ curl -X GET http://coder-server:8080/api/v2/deployment/config \
         "retention": 0,
         "send_actor_headers": true,
         "structured_logging": true
+      },
+      "chat": {
+        "acquire_batch_size": 0
       }
     },
     "allow_workspace_renames": true,