docs(UAT_PLAYBOOK): add TC-API-3.28 for pet_size_category enum verification (GRO-1999)

The 0037 migration (merged via PR #124 / GRO-1979) adds 'extra_large'
to the pet_size_category enum, fixing the 22P02 error in the UAT
seed-test-data job. Mirror the regression test added for coat_type
in TC-API-3.27: TC-API-3.28 explicitly verifies pet_size_category
contains all 4 values used by seed.ts  after
the seed job completes, so future enum drift on either dimension
surfaces at the UAT gate.

Refs: GRO-1999, GRO-1971 (precedent), PR #124 (the code fix).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

UAT_PLAYBOOK.md

@@ -119,6 +119,7 @@ GroomBook API is a Hono-based REST service (TypeScript/Node.js) powering the pet
 | TC-API-3.25 | Verify 30+ total pets in UAT DB | GET /api/pets then count total | 30+ pets returned (UAT seed creates 500 random-pool + 5 UAT test clients + 2 UAT customer = 507 total) |
 | TC-API-3.26 | Verify 25-35% medicalAlerts distribution | GET /api/pets (first 30 pets), count how many have non-empty medicalAlerts | Ratio is 25-35% (seed uses rand() < 0.3 for ~30% distribution) |
 | TC-API-3.27 | Verify coat_type enum has all seed values | After UAT seed completes, inspect the coat_type enum on the UAT DB — it must contain: short, medium, long, double, wire, silky, curly, hairless | UAT seed jobs (`reset-demo-data`, `seed-test-data`) complete 1/1 with no `enum_in` error; coat_type includes all 8 values used by seed.ts `coatTypePool` |
+| TC-API-3.28 | Verify pet_size_category enum has all seed values | After UAT seed completes, inspect the pet_size_category enum on the UAT DB — it must contain: small, medium, large, extra_large | UAT seed jobs (`reset-demo-data`, `seed-test-data`) complete 1/1 with no `enum_in` error; pet_size_category includes all 4 values used by seed.ts `petSizeCategoryPool` (regression for GRO-1999, mirrors TC-API-3.27) |
 
 ### 4.4 Appointment Scheduling