sync_creatives should not create assignments for creatives that failed validation

[bokelley]

From local storefront E2E against ghcr.io/bokelley/salesagent:latest (image id 8d9a7d94b344), sync_creatives can return INTERNAL_ERROR when a creative fails validation but the request also includes assignments.

Repro shape:

• create_media_buy succeeds and returns a package.
• sync_creatives sends one new creative with an unsupported format_id.agent_url, plus an assignment to the package.
• Creative validation fails, so no row is inserted into creatives.
• _process_assignments still inserts creative_assignments for that creative.

Observed DB error:

psycopg2.errors.ForeignKeyViolation: insert or update on table "creative_assignments" violates foreign key constraint "fk_creative_assignments_creative_composite"
DETAIL: Key (creative_id, tenant_id, principal_id)=(strict-e2e-creative-..., tenant_5b0bec3d, principal_1951e7c9) is not present in table "creatives".

Expected:

• If a creative fails validation, assignment processing should skip it and return a per-creative action: failed response with an assignment/validation error.
• No FK violation or INTERNAL_ERROR should leak to the buyer.

Likely area: src/core/tools/creatives/_sync.py processes assignments after creative processing, and _assignments.py does not appear to check that the creative exists before upserting the assignment.