fix: handle missing relationship descriptions in attackToExcel

[jondricek]

Summary

• make relationship citation extraction tolerate relationship dataframes that do not contain a mapping description column
• preserve existing citation behavior for relationships that do include descriptions and citation markers
• add a regression test covering an in-memory ICS bundle with an asset targets relationship that has no description

Root Cause

relationshipsToDf() only populated mapping description when a relationship had a description, but later citation extraction assumed the column always existed and indexed it unconditionally. ICS asset targets relationships can be valid without a description, which caused a KeyError during Excel export.

Test Plan

• uv run pytest tests/test_stix_to_df.py
• uv run ruff check mitreattack/attackToExcel/stixToDf.py tests/test_stix_to_df.py
• uv run ruff format --check mitreattack/attackToExcel/stixToDf.py tests/test_stix_to_df.py

[sonarqubecloud]

Quality Gate failed

Failed conditions
C Reliability Rating on New Code (required ≥ A)

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