fix(opencode): backward compatibility for old sessions with string an…

[bagatka]

Fixes: #7454 #7438 #7433 #7392

What does this PR do?

• Fixes a crash when opening sessions created before the multi-select question feature (#7386)
• Adds backward compatibility for both old (string) and new (string[]) answer formats in the question tool

How did you verify your code works?

Created a session with question answers using an older version of opencode (before 1.1.8), then updated to 1.1.8 and confirmed the crash when opening the session. After checking out this branch, the session loads correctly.