Motivation & Context

add_items signals both a missing conversation and unsupported item content with a plain ValueError, and the route answers every ValueError with 404. Posting a video-only item to an existing conversation therefore reported Not Found, which reads as a wrong conversation id rather than a request the client can fix.

Description & Review Guide

• What are the major changes? Unsupported item content now raises a ConversationItemValidationError (a ValueError subclass), and the items route maps it to 400 while a missing conversation stays 404.
• What is the impact of these changes? Only the status code for the malformed item case changes. Other handlers catching ValueError are unaffected because the new exception subclasses it.
• What do you want reviewers to focus on? The route test covers both cases against the HTTP surface: unsupported content on an existing conversation is 400, and an unknown conversation id is still 404.

Related Issue

Fixes #9506

AI Assistance

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