Add message.get_conversation_index() to pypff

[kimberlyz]

Summary

This PR adds support for retrieving the MAPI PR_CONVERSATION_INDEX property from message objects in the Python bindings.

Motivation

The conversation index is a critical property for email threading and conversation tracking in Microsoft Outlook/Exchange PST files. This property was previously inaccessible through pypff, making it difficult to reconstruct email conversation threads or perform thread-based analysis of PST files.

Changes Made

Core Implementation (pypff/pypff_message.c, pypff/pypff_message.h):

• Added pypff_message_get_conversation_index() function
• Returns raw binary data as bytes (Python 3) or str (Python 2)
• Returns None if the property doesn't exist
• Exposed as both method message.get_conversation_index() and property message.conversation_index

API Usage

import pypff

pff_file = pypff.file()
pff_file.open("sample.pst")
message = pff_file.get_root_folder().get_sub_message(0)

# Method access
conv_index = message.get_conversation_index()  # Returns bytes or None

# Property access
conv_index = message.conversation_index  # Returns bytes or None

pff_file.close()

Technical Notes

• Uses existing libpff_message_get_conversation_index() and libpff_message_get_conversation_index_size() functions from the C library
• Follows the same pattern as other property getters in pypff_message.c