### Issue:
When a custom button snippet was dropped next to an existing button 
wrapped in a `<div>`, it was incorrectly and unconditionally wrapped in 
a paragraph `<p>` tag. This issue did not occur with standard button 
snippets or when the buttons were wrapped in a `<p>` tag.

### Steps to reproduce:
1. Drag a standard "Button" snippet next to an existing button (e.g., in
   the navbar). The structure is correctly `<a/><a/>`.
2. Save a button as a custom snippet.
3. Drag this new custom button snippet next to a button wrapped in a 
   `<div>` (navbar in our example).
4. Observe the structure: 
   `<div><a/><p><a class="s_custom_snippet"/></p></div>`,
   but it should be like `<div><a/><a class="s_custom_snippet"/></div>`.

### Fix
The logic has been corrected to only wrap a button snippet in a 
paragraph tag if it has no sibling buttons and is not already inside a 
`<p>`. This ensures consistent behaviour between standard and 
custom button snippets. A tour test is also added, ensuring the 
behaviour is correct.

Backport of https://github.com/odoo/odoo/commit/3a8f2cc418510c3b91f8b9086bb4a89a1745e40d

task-[5032130](https://www.odoo.com/odoo/project/974/tasks/5032130)