ENG-374: Update add_labels and remove_labels to accept string or array

[josephfeleke]

add_labels and remove_labels in UpdateMessageRequest now accept either a single string or a list of strings, matching the API change merged in agentmail-to/agentmail-api.

Added UpdateMessageLabels union type (following the existing Addresses pattern) and updated both fields to use it.

[github-actions]

✨ API Changes

# API Changelog n/a vs. n/a

## POST /v0/inboxes/{inbox_id}/drafts/{draft_id}/send
- :warning: the request property 'add_labels' became not nullable
- :warning: the request property 'remove_labels' became not nullable
-  request property 'add_labels' list-of-types was widened by adding types 'string' to media type 'application/json'
-  request property 'remove_labels' list-of-types was widened by adding types 'string' to media type 'application/json'


## PATCH /v0/inboxes/{inbox_id}/messages/{message_id}
- :warning: the request property 'add_labels' became not nullable
- :warning: the request property 'remove_labels' became not nullable
-  request property 'add_labels' list-of-types was widened by adding types 'string' to media type 'application/json'
-  request property 'remove_labels' list-of-types was widened by adding types 'string' to media type 'application/json'

💡 Download api-changelog-diff artifact or tag @fern Writer in #github-prs for changelog.

[github-actions]

🌿 Preview your docs: https://agentmail-preview-d4181997-4c97-4cf6-b609-7789b48dc1f5.docs.buildwithfern.com

[cubic-dev-ai]

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