Pull Request Template

Description

Added the possibility to mark as a single conversation in the API type inbox. This allows the conversation builder to search for the last conversation.
I thought about searching for the last conversation with created_at: desc order, as is done in some channels... but I didn't change the way the conversation is searched.

Fixes # (issue)
#7726

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)
• This change requires a documentation update

How Has This Been Tested?

Added test case in conversation_builder_spec;

I started a new conversation via API, sent some messages and ended it.
I started another conversation in the same channel, and verified that the conversation remained the same.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules