fix: only write AI model prices that changed (#27923)

Previously, the AI Gateway price seeder rewrote every row of
`ai_model_prices` on each server start, because `ON CONFLICT` fires on a
key conflict rather than on a value difference. `updated_at` therefore
recorded when the server last restarted rather than when a price last
changed.

Guard the `DO UPDATE` branch so a conflicting row is only rewritten when
one of its four prices differs. The comparison uses `IS DISTINCT FROM`
rather than `<>` because the price columns are nullable, and `<>` yields
NULL when either side is NULL, which would skip the update and leave a
stale price in place.

Related to
https://linear.app/codercom/issue/AIGOV-567/experimental-cli-command-to-set-prices-for-unpriced-ai-models

> [!NOTE]
> Initially generated by Claude Opus 5, modified and reviewed by
@ssncferreira

coderd/database/queries.sql.go

@@ -3459,11 +3459,25 @@ ON CONFLICT (provider, model) DO UPDATE SET
 	cache_read_price  = EXCLUDED.cache_read_price,
 	cache_write_price = EXCLUDED.cache_write_price,
 	updated_at        = NOW()
+WHERE (
+	ai_model_prices.input_price,
+	ai_model_prices.output_price,
+	ai_model_prices.cache_read_price,
+	ai_model_prices.cache_write_price
+) IS DISTINCT FROM (
+	EXCLUDED.input_price,
+	EXCLUDED.output_price,
+	EXCLUDED.cache_read_price,
+	EXCLUDED.cache_write_price
+)
 `
 
 // Upsert a batch of (provider, model) rows from a JSON array. Each element
 // must have provider, model, and the four price fields; null prices are
 // written as SQL NULL.
+// A conflicting row is only rewritten when a price differs, so updated_at
+// records when a price last changed. Prices are nullable and a NULL on
+// either side counts as a difference.
 func (q *sqlQuerier) UpsertAIModelPrices(ctx context.Context, seed json.RawMessage) error {
 	_, err := q.db.ExecContext(ctx, upsertAIModelPrices, seed)
 	return err