Fix golangci-lint: formatting, nilerr false positive, deprecated linter names

- gofmt: fix struct field alignment in types.go, resource_server.go,
  data_source_ssh_keys_by_user.go
- nilerr: refactor GetAllPages pagination detection to avoid returning
  nil error when json.Unmarshal fails (intentional passthrough for
  non-paginated responses)
- .golangci.yml: replace deprecated linter names (vet -> govet,
  tenv -> usetesting)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/provider/data_source_ssh_keys_by_user.go

@@ -32,8 +32,8 @@ type SSHKeysByUserDataSource struct {
 
 // SSHKeysByUserDataSourceModel describes the data source data model.
 type SSHKeysByUserDataSourceModel struct {
-	UserID  types.Int64            `tfsdk:"user_id"`
-	Results types.Int64            `tfsdk:"results"`
+	UserID  types.Int64             `tfsdk:"user_id"`
+	Results types.Int64             `tfsdk:"results"`
 	SSHKeys []SSHKeyByUserItemModel `tfsdk:"ssh_keys"`
 }