fix(plugins): composer option resets and model checks match the documented commands

A draft kept `null` as an option value, so `setOptions({ key: null })` sent the null to the
provider and creation failed. A null edit now removes the key from every draft layer, as the patch
does on a running agent. A running agent's `setModel` accepted ids its catalog does not list; it
now rejects them the way a draft does.

The composer controls imported the toast host through the toast context, which pulled the host's
animation module into every module that loads plugins. They use the API-only context instead.
The migration guide's registration table gets the `addComposerItem` row its test requires.

public-docs/plugins/migration.md

@@ -80,6 +80,7 @@ Use this table as the complete registration checklist.
 | `plugin.addClientSlashCommand(command)` in the old root entry                                 | `client.addSlashCommand(command)` in `index.client.tsx`                                                                     |
 | `plugin.addClientSide(fn)` in the old root entry                                              | Delete the wrapper and move the body of `fn` into the default client entry function                                         |
 | `client.addComposerPill(pill)` inside the old client callback                                 | `client.addComposerPill(pill)` inside `index.client.tsx` or an imported `client/` function                                  |
+| New composer item contribution                                                                | `client.addComposerItem(item)` in `index.client.tsx`; see [composer items](reference#composer-items)                        |
 | New header contribution                                                                       | `client.addHeaderButton({ id, workspaceId, button })`                                                                       |
 | `plugin.addAttachmentSource(source)` in the old root entry                                    | `client.addAttachmentSource(source)` in `index.client.tsx`                                                                  |
 | New settings screen contribution                                                              | `client.addSettingsScreen(screen)` in `index.client.tsx`; see [settings screens](reference#settings-screens)                |

public-docs/plugins/migration.zh-CN.md

@@ -73,6 +73,7 @@ my-plugin/
 | 旧根入口中的 `plugin.addClientSlashCommand(command)`                                          | `index.client.tsx` 中的 `client.addSlashCommand(command)`                                                              |
 | 旧根入口中的 `plugin.addClientSide(fn)`                                                       | 删除这层包装，把 `fn` 的函数体移入默认的客户端入口函数                                                                 |
 | 旧客户端回调中的 `client.addComposerPill(pill)`                                               | `index.client.tsx` 或被导入的 `client/` 函数中的 `client.addComposerPill(pill)`                                        |
+| 新的编辑器项贡献                                                                              | `index.client.tsx` 中的 `client.addComposerItem(item)`;见[编辑器项](reference#composer-items)                          |
 | 新的标题栏贡献                                                                                | `client.addHeaderButton({ id, workspaceId, button })`                                                                  |
 | 旧根入口中的 `plugin.addAttachmentSource(source)`                                             | `index.client.tsx` 中的 `client.addAttachmentSource(source)`                                                           |
 | 新的设置界面贡献                                                                              | `index.client.tsx` 中的 `client.addSettingsScreen(screen)`；见[设置界面](reference#settings-screens)                   |