Fix: IndexError when replaying incomplete trajectories

[klieret]

Closes #124

[codecov]

Codecov Report

Attention: Patch coverage is 36.36364% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 75.83%. Comparing base (2d0caa5) to head (1dd10b5).
Report is 5 commits behind head on main.

Files	Patch %	Lines
sweagent/agent/models.py	36.36%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #410      +/-   ##
==========================================
- Coverage   75.94%   75.83%   -0.11%     
==========================================
  Files          18       18              
  Lines        2885     2893       +8     
==========================================
+ Hits         2191     2194       +3     
- Misses        694      699       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.