Validate channel_id as UUID in remove_self endpoint

[ashnaaseth2325-oss]

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Target branch: develop

Observed behavior

The remove_self endpoint in ChannelUserViewSet does not explicitly validate channel_id as a UUID before querying the database.

When a malformed UUID (e.g., not-a-valid-uuid) is provided, it results in an unhandled exception and results in a 500 Internal Server Error response.

Errors and logs

ValueError: badly formed hexadecimal UUID string

User-facing consequences

Users receive a 500 Internal Server Error when providing invalid input instead of a proper validation error (400 Bad Request). This leads to a poor user experience and unclear API behavior.

Steps to reproduce

1. Call the remove_self endpoint
2. Provide an invalid channel_id (e.g., not-a-valid-uuid)
3. Observe that the response is a 500 error instead of a 400 Bad Request

Context

• Application version: current develop branch
• Operating system: any
• Browser: any

AI usage

I used AI assistance to help structure and refine this issue description. The issue itself was identified, reproduced, and validated manually.