Warn when a desktop message is missing frequency guardrails #17662

Tentative, pending review by OMC. Per discussion with OMC, the rules for messaging guardrails should come from OMC; the list below is a starting point for that review, not a final spec.

Desktop messages that ship without a frequency group or with missing or extreme lifetime limits can show more often than intended, or collide with other messages in the same surface. Recipe owners add these by hand today and they are often missed. OMC tracks the Firefox-side schema work in OMC-1969 and the limits definition in OMC-1970. Experimenter can already read groups and frequency from the message JSON, so a first version does not need to wait for the schema.

Proposal

Add non-blocking review warnings for desktop deliveries with a messaging feature, for each message:

• no frequency group with an interval limit (for example cfr);
• no frequency.lifetime, or a lifetime above an agreed maximum;
• message types OMC identifies as needing a specific group (for example default-browser prompts) that are not in it.

Acceptance criteria

• Rules and thresholds agreed with OMC, with example messages that pass and fail.
• Non-blocking warnings on the review warning card naming the feature and message ids.

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