fix: validate unique tag IDs for DFP Async provider

DFP Async uses the tag_id field as a div ID in the rendered HTML. When
two ad codes share the same tag_id, Google's ad serving becomes confused
because the same element ID appears multiple times on the page.

This adds validation that prevents saving ad codes with duplicate tag_id
values. The implementation uses a new 'acm_validate_ad_code' filter that
providers can hook into for custom validation, keeping the solution
extensible for other providers that may need similar checks.

Fixes #69

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Co-Authored-By: Claude <[email protected]>

Author: GaryJones