Refactor: conversation_export accesses private chat_history_manager methods

[mrveiss]

Discovered During

Code review of PR #3176 (#1808 — conversation export/import).

Problem

`_load_full_session_data()` in `services/conversation_export.py` accesses private methods:

• `chat_history_manager._get_chats_directory()`
• `chat_history_manager._decrypt_data()`

This creates tight coupling to internal implementation details of the chat history manager.

Fix

Add a public method to `ChatHistoryManager` (e.g., `load_full_session(session_id)`) that returns the complete session data including metadata, then use it in the export service.