refactor: improve code structure and maintainability

[linw1995]

Summary

Refactor code to improve maintainability by extracting common helper functions and simplifying complex logic.

Main Changes

• Unified error handling: Extract helper functions like invalid_error(), reply_validation_error(), reply_validation_success() to reduce boilerplate
• Extract common logic:
  • agent.rs: Extract build_chat_response_from_blocks() method to reduce duplication between chat() and chat_with_history()
  • tools.rs: Add parse_relative_path() helper with unit tests
  • mcp.rs: Extract serialize_to_json_value(), parse_call_tool_payload(), require_server_for_action() helpers
  • combo/runner.rs: Extract emit_prompt_stream_update() to simplify stream update handling
  • combo/starter.rs: Extract helper functions for reply validation
• Retry mechanism improvement: provider.rs uses complete_with_retry_status() wrapper for consistent retry status handling
• Code organization: Move sanitize_log_stem() to logging module for better reusability

[codecov-commenter]

Codecov Report

❌ Patch coverage is 22.35529% with 389 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.53%. Comparing base (55394f6) to head (521582e).
⚠️ Report is 4 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/provider.rs	0.00%	161 Missing ⚠️
src/combo/runner.rs	0.00%	77 Missing ⚠️
src/combo/starter.rs	20.83%	76 Missing ⚠️
src/agent.rs	0.00%	35 Missing ⚠️
src/agent/prompt.rs	20.68%	23 Missing ⚠️
src/mcp.rs	85.71%	7 Missing ⚠️
crates/coco-tui/src/components/chat.rs	0.00%	6 Missing ⚠️
src/tools.rs	90.47%	2 Missing ⚠️
src/config/env.rs	75.00%	1 Missing ⚠️
src/logging.rs	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #180      +/-   ##
==========================================
- Coverage   48.12%   47.53%   -0.59%     
==========================================
  Files         105      108       +3     
  Lines       27916    30135    +2219     
==========================================
+ Hits        13435    14326     +891     
- Misses      14481    15809    +1328     

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