WPB-28421 add an opt in policy for dropping unsupported federated notifications

[battermann]

https://wearezeta.atlassian.net/browse/WPB-28421

Checklist

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

[Copilot]

Pull request overview

This PR introduces an opt-in per-message policy for how the background worker should handle federated backend-notification bundles when there is no common federation API version with the target backend. It extends the RabbitMQ payload format in a backward-compatible way (defaulting to the safe “keep queued” behavior) while enabling producers to explicitly request dropping unsupported notifications.

Changes:

• Add UnsupportedVersionPolicy and extend PayloadBundle with an optional unsupportedVersionPolicy field (defaults to KeepQueued on decode).
• Update the background worker pusher to either keep the message queued (and mark the queue as stuck) or ack+count-drop based on the policy.
• Add Prometheus metric wire_backend_notifications_dropped_unsupported_version and extend tests to cover default/combination behavior and drop-vs-keep behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
services/background-worker/test/Test/Wire/BackendNotificationPusherSpec.hs	Adds coverage for payload bundle defaulting/semigroup behavior and pushNotification behavior for unsupported versions under both policies.
services/background-worker/src/Wire/BackgroundWorker/Env.hs	Adds a new Prometheus counter for dropped-unsupported-version notifications.
services/background-worker/src/Wire/BackendNotificationPusher.hs	Implements policy-driven behavior when no compatible API version exists (keep queued vs drop+ack), and updates metrics/gauge accordingly.
libs/wire-api-federation/src/Wire/API/Federation/BackendNotifications.hs	Introduces UnsupportedVersionPolicy, extends PayloadBundle schema with a backward-compatible optional field, and defines safe Semigroup behavior.
changelog.d/6-federation/WPB-28421	Changelog entry for the new opt-in drop policy.

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[akshaymankar]

Why default to KeepQueued? Looks like previous idea was to ignore the notification?

[battermann]

Because this is explicitly an opt-in policy. If the default was to ignore/drop we would risk a state drift between remotes.