refactor: remove redundant combo reply error variant and add test coverage

[linw1995]

No description provided.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 51.06%. Comparing base (e5977ab) to head (505daae).
⚠️ Report is 6 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/coco-tui/src/components/chat.rs	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #103      +/-   ##
==========================================
- Coverage   53.46%   51.06%   -2.40%     
==========================================
  Files          88       91       +3     
  Lines       21248    23329    +2081     
==========================================
+ Hits        11360    11913     +553     
- Misses       9888    11416    +1528     

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