Steps to reproduce:
-------------------------
1. Install Website and AI modules.
2. Open the website editor.
3. From the Intro block, search for the AI snippet and add the snippet contains 'Need Help?'.
4. Click on the added block, then click the Save icon from the Style tab.
5. Save the snippet
6. Go to Blocks → Custom Blocks and add the saved snippet.

Observation:
-------------------------
A traceback is raised:
```
TypeError: Cannot read properties of null (reading 'remove')
    at AILivechatOptionPlugin.onSnippetDropped
```

Issue:
-------------------------
The `onSnippetDropped` method
https://github.com/odoo/enterprise/blob/bb3699cc112acb3ec545d3aa5f3d8f7263a5dac6/ai_website_livechat/static/src/website/plugins/ai_livechat_option_plugin.js#L39-L51 executes every time a snippet is dropped into the layout. When the snippet is first added, the method removes the `<div>` with class `s_ai_livechat_preview`. Upon saving, the snippet is stored without that `div`. When re-adding the saved snippet from custom tab, the method runs again but fails since the `s_ai_livechat_preview` element no longer exists, resulting in the traceback.

Solution:
-------------------------
Added the optional chaining operator to safely remove the `s_ai_livechat_preview` div only when it exists.

opw-5227072

Forward-Port-Of: odoo/enterprise#99276

Before this commit, "Copy to Clipboard" was sometimes not visible in the message action list when message had some text content.

This happens because message actions can condition overridden by some other modules, and the way overrides of action condition work is to have boolean value taking precedence over the local condition of action.

This architecture is meant to provide exhaustive list of actions to allow, which is quite useful to control what livechat visitors can have.

However this architecture requires overrides to properly return `undefined` when the local condition should apply, which was not the case for an override by `ai` module. So when `ai` module is installed, the "copy to clipboard" feature as not available.

This commit fixes the issue by adapting the condition in `ai` module to return `undefined` on non-AI conversations, and add test coverage for both `@mail` discuss suite and all overrides of discuss in `@test_discuss_full_enterprise`.

opw-5240768

https://github.com/odoo/odoo/pull/236295

Forward-Port-Of: odoo/enterprise#99794