Fix for multi-segment issue

[jegan-maker]

Problem

Multi-segment downloads (especially C53) were failing because segments were being decrypted individually instead of being combined first.

Solution

This PR fixes the multi-segment download process to match EBICS specification:

1. Extract and decrypt transaction key from first segment only
2. Collect all segments as encrypted binary data (without individual decryption)
3. Combine all encrypted segments
4. Decrypt the combined data using the cached transaction key
5. Decompress the result

Changes

• Added order_data_binary method to get raw encrypted segment data
• Modified download method to handle multi-segment responses correctly
• Added transaction key caching across segments
• Improved debug logging

Testing

• Tested with C53 multi-segment downloads
• Verified backward compatibility with single-segment downloads
• All existing tests pass

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