Deprecate adGroupDetails.childAdType in favor of adDetails.adType on individual ads

## Problem

`xdm:childAdType` on `adGroupDetails` is redundant and inaccurate:

1. **Redundant**: `adDetails.adType` already exists on each individual ad in the Ad Lookup schema with a full enum of 35+ values.
2. **Inaccurate**: `childAdType` is a single `string` — it cannot represent ad groups that contain mixed ad types (common on Meta, TikTok, and other platforms). Setting it for heterogeneous groups requires leaving it blank or using `"other"`, making the field useless.

## Solution

Soft-deprecate `xdm:childAdType` by:
- Updating the `description` to indicate it is deprecated and point consumers to `adDetails.adType`
- Adding `"meta:status": "deprecated"`

## Impact

Consumers should use `adDetails.adType` on each ad record in the Ad Lookup to determine ad type, which handles mixed-type ad groups correctly.

Search the schema definitions for xdm:childAdType and inspect its current description and metadata. Mark the field deprecated, point consumers to adDetails.adType, and run the repository's schema validation checks to confirm the updated definition remains valid.

Written by the indexing model from the issue text.