Backend/notifs: Remove deprecated notification_data.ChatMessage.message

[tristanlabelle]

It was kept as a fallback in #8753 (merged 2026-05-26) because we might still have been processing jobs referencing ChatMessage instances with a message value. I think it's safe to assume those have drained by now.

Testing

Updated the test that used this field. Otherwise no test covered the deprecated code path.

Backend checklist

• Added tests for any new code or added a regression test if fixing a bug
• Run the backend locally and it works
• Added migrations if there are any database changes, rebased onto develop if necessary for linear migration history

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

[CouchersBot]

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Backend

Schema	Schema diff	Sample emails

Other

Protos	Backend coverage	Web coverage

[github-actions]

📝 Release Notes

This PR does not need to be included in release notes.

Reason: This PR removes deprecated backend fallback code and reserves an old proto field after previous migration work. It is an internal cleanup with no new user-facing feature, noticeable UX change, or significant end-user impact on its own.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This PR removes deprecated backend fallback code and reserves an old proto field after previous migration work. It is an internal cleanup with no new user-facing feature, noticeable UX change, or significant end-user impact on its own.