Warn when a desktop delivery uses deprecated fxms-message-N placeholder features

Problem

Firefox 152 added a single co-enrolling desktop Nimbus feature fxms-message (allowCoenrollment: true), which replaces the 25 individual non-co-enrolling placeholder features fxms-message-1 through fxms-message-25. Added upstream by Bug 1961268, mozilla-central 455b1fe98c5f. From that commit: "There's no need to have more than one, since it won't behave any differently."

The numbered placeholders still exist and still work, so nothing is broken, but a delivery that only targets versions where fxms-message is available should use it. The numbered features each occupy a single feature slot, so two deliveries using the same numbered ID cannot both enroll a client, while fxms-message allows any number of simultaneous enrollments.

Nothing currently tells an author this.

Proposed change

Add a non-blocking review warning, not a launch-blocking error, on the existing shared warning card. #17087 made this a serializer-only change.

Add _validate_desktop_fxms_message_coenrollment to NimbusReviewSerializer, dispatched from the existing desktop block at the end of validate(). It sets self.warnings["fxms_message_coenrollment"] when a desktop delivery attaches a numbered placeholder AND the co-enrolling replacement is available across the delivery's entire targeted version range.

Determine availability from the manifest data rather than hardcoding a version number. Experimenter already ingests this: allow_coenrollment is a field on NimbusVersionedSchema populated by load_feature_configs, and fxms-message is absent from every versioned manifest below v152.0.0. So NimbusFeatureConfig.get_versioned_schema_range(min, max) answers the question directly, and the check stays correct if the manifest changes.

Control flow:

1. Collect attached feature configs whose slug starts with fxms-message-. None, return.
2. Return if firefox_min_version is below MIN_VERSIONED_FEATURE_VERSION[DESKTOP]. Below that floor get_versioned_schema_range falls back to the unversioned schema and would wrongly report the replacement as available.
3. Look up the fxms-message feature config. Absent, return.
4. Warn only if the range yields schemas, has no unsupported versions, and every schema allows co-enrollment.

Also add a REVIEW_WARNING_LABELS entry so the card shows a readable label rather than the humanized fallback.

Acceptance criteria

• A desktop delivery attaching a numbered placeholder, whose whole targeted range supports fxms-message, shows a non-blocking warning naming the offending feature IDs and the replacement.
• No warning when the targeted range includes versions without fxms-message.
• No warning when no numbered placeholder is attached.
• The warning never blocks launch or review.
• Non-desktop applications are unaffected.

Notes

Follow-up to #17087, which built the shared warning path this uses.

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