Fix missing partial when editing comments with @mentions

[flavorjones]

Summary

• Edge Rails introduced ActionText::Attachable#to_editor_content_attachment_partial_path which defaults to to_partial_path ("users/user"). When editing a comment containing @mentions, the new editor adapter system calls this method to render mention content inside the rich text editor. Since users/_user.html.erb doesn't exist, this raises ActionView::Template::Error.
• Override to_editor_content_attachment_partial_path in User::Mentionable to delegate to to_attachable_partial_path ("users/attachable"), matching the existing display rendering path.

The Rails change was introduced as part of the edge Rails bump in #2820 (Lexxy 0.9.5.beta), which incidentally upgraded Rails from 12e24eaf to 75f9e28379ac. The new to_editor_content_attachment_partial_path method was added as part of the editor adapter system. See the upgrade analysis for full details on changes in this Rails bump.

Test plan

• Regression test: edit a comment containing an @mention, assert the editor renders the mention attachment with the correct SGID and user name
• Verified test fails without the fix (Missing partial users/_user)
• Verified test passes with the fix
• All existing comment controller tests pass

[Copilot]

Pull request overview

This PR fixes a regression introduced by the Edge Rails editor adapter system when rendering @mention attachments inside the comment editor: Rails now calls to_editor_content_attachment_partial_path, which previously defaulted to a non-existent users/_user partial for User mentions.

Changes:

• Override User#to_editor_content_attachment_partial_path (via User::Mentionable) to use the existing users/attachable rendering path.
• Add a controller regression test covering editing a comment that contains an @mention attachment.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
app/models/user/mentionable.rb	Adds an explicit editor-attachment partial path override to prevent missing-partial errors during comment editing.
test/controllers/cards/comments_controller_test.rb	Adds regression coverage asserting mention attachments render correctly in the editor’s initial value.

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