fix(coderd): fix error handling in deleteUserWebpushSubscription (#22500)

## Summary

`deleteUserWebpushSubscription` in `coderd/webpush.go` had incorrect
error handling that masked database errors as 404 responses.

## Bug

`GetWebpushSubscriptionsByUserID` is a `:many` query — it returns `([],
nil)` when no rows match, never `sql.ErrNoRows`. The previous `if/else
if` chain:

```go
if existing, err := api.Database.GetWebpushSubscriptionsByUserID(ctx, user.ID); err != nil && errors.Is(err, sql.ErrNoRows) {
    // dead code — :many queries never return sql.ErrNoRows
} else if idx := slices.IndexFunc(existing, ...); idx == -1 {
    // real DB errors fall through here, existing is nil, idx is -1 → 404
}
```

Any real database error (connection failure, timeout, authorization
error) fell through to the `else if` branch where `slices.IndexFunc(nil,
...)` returns `-1`, returning 404 "subscription not found" instead of
500.

## Fix

Split into two separate checks so database errors properly return 500:

```go
existing, err := api.Database.GetWebpushSubscriptionsByUserID(ctx, user.ID)
if err != nil {
    // 500
}
if idx := slices.IndexFunc(existing, ...); idx == -1 {
    // 404
}
```

## Testing

Added `TestDeleteWebpushSubscription/database_error_returns_500` which
wraps the DB store to inject an error into
`GetWebpushSubscriptionsByUserID` and asserts the handler returns 500
(not 404).

coderd/webpush.go

@@ -88,12 +88,15 @@ func (api *API) deleteUserWebpushSubscription(rw http.ResponseWriter, r *http.Re
 	}
 
 	// Return NotFound if the subscription does not exist.
-	if existing, err := api.Database.GetWebpushSubscriptionsByUserID(ctx, user.ID); err != nil && errors.Is(err, sql.ErrNoRows) {
-		httpapi.Write(ctx, rw, http.StatusNotFound, codersdk.Response{
-			Message: "Webpush subscription not found.",
+	existing, err := api.Database.GetWebpushSubscriptionsByUserID(ctx, user.ID)
+	if err != nil {
+		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Failed to get webpush subscriptions.",
+			Detail:  err.Error(),
 		})
 		return
-	} else if idx := slices.IndexFunc(existing, func(s database.WebpushSubscription) bool {
+	}
+	if idx := slices.IndexFunc(existing, func(s database.WebpushSubscription) bool {
 		return s.Endpoint == req.Endpoint
 	}); idx == -1 {
 		httpapi.Write(ctx, rw, http.StatusNotFound, codersdk.Response{