refactor: More clearly separate writer from temporary file

We now should be more explicit about what is the context in which the
batch export temporary file is alive. The writer outlives this
context, so it can be used by callers to, for example, check how many
records were written.

posthog/temporal/batch_exports/s3_batch_export.py

@@ -20,8 +20,10 @@
 from posthog.temporal.batch_exports.base import PostHogWorkflow
 from posthog.temporal.batch_exports.batch_exports import (
     CreateBatchExportRunInputs,
+    FlushCallable,
     JSONLBatchExportWriter,
     ParquetBatchExportWriter,
+    UnsupportedFileFormatError,
     UpdateBatchExportRunStatusInputs,
     create_export_run,
     default_fields,