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/querier.go

@@ -1611,6 +1611,9 @@ type sqlcQuerier interface {
 	// 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.
 	UpsertAIModelPrices(ctx context.Context, seed json.RawMessage) error
 	// Returns true if a new rows was inserted, false otherwise.
 	UpsertAISeatState(ctx context.Context, arg UpsertAISeatStateParams) (bool, error)