Session export/import better preserves tool-use messages

What

• When rebuilding an assistant message during session export/import, stop_reason is no longer always set to end_turn; it is now set to tool_use when the message actually contains tool-use blocks.
• When importing an assistant message, file-write tool calls are now reconstructed and re-added as tool_use blocks (with their file_path and content), in addition to the existing text-block handling.

Why Previously, exporting and re-importing a session could lose or mislabel tool-use information, such as file writes a session had made. This keeps transcripts more faithful when moved between sessions or environments.