WPB-23789: Migrate ensureAllowed to type class for Galley/Action

[blackheaven]

https://wearezeta.atlassian.net/browse/WPB-23789

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[battermann]

Please consider my comment, otherwise LGTM

[battermann]

I'd suggest to rebase and update the changelog by just appending the PR number here.

[Copilot]

Pull request overview

This PR refactors Galley conversation-action authorization by moving ensureAllowed from a tag-dispatch function into the IsConversationAction type class, so each action’s authorization logic lives alongside its performAction implementation.

Changes:

• Add ensureAllowed as a method on IsConversationAction and implement it per action tag.
• Update call sites to use the typeclass method (dropping the explicit Sing tag argument).
• Update internal changelog entry to mention the ensureAllowed migration.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
services/galley/src/Galley/API/Update.hs	Updates the channel member-removal path to call the new typeclass-based ensureAllowed.
services/galley/src/Galley/API/Action.hs	Introduces ensureAllowed as a typeclass method, adds per-action implementations, and updates updateLocalConversationUnchecked to call the method.
changelog.d/5-internal/WPB-23789	Notes the migration of ensureAllowed in the refactoring changelog entry.

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