fix: handle large tool outputs and API errors gracefully

[linw1995]

• Add error response parsing in Anthropic client to provide better error messages
• Implement tool output truncation to prevent context overflow (80KB limit)
• Add proper error handling for chat API failures in TUI components
• Fix message locking issue in agent to avoid deadlock scenarios
• Add comprehensive tests for truncation logic and error handling

[codecov-commenter]

Codecov Report

❌ Patch coverage is 46.59686% with 102 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.14%. Comparing base (fad69e0) to head (e9ff67a).
⚠️ Report is 3 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/coco-tui/src/components/chat.rs	55.97%	70 Missing ⚠️
crates/anthropic/src/client/messages.rs	0.00%	18 Missing ⚠️
src/agent.rs	0.00%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #74      +/-   ##
==========================================
- Coverage   59.25%   59.14%   -0.11%     
==========================================
  Files          77       77              
  Lines       15488    15666     +178     
==========================================
+ Hits         9177     9266      +89     
- Misses       6311     6400      +89     

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