refactor: reduce code duplication with macros and convenience helpers

[linw1995]

Summary

This PR refactors error handling and request retry logic to reduce code duplication across the codebase.

Changes

• Extract fail_early! macro in src/combo/starter.rs: Reduces repetitive error handling code that sends failed events and returns early
• Extract with_retry! macro in both crates/anthropic and crates/openai: Consolidates retry logic with backoff, notification, and error transformation
• Add convenience helper require_from_env() in src/combo/session.rs: Simplifies session socket client initialization with proper error handling
• Add ChatMessage constructor methods in crates/openai/src/types.rs: Provides system(), user(), assistant(), and tool_result() static methods for cleaner message creation

[codecov-commenter]

Codecov Report

❌ Patch coverage is 19.64286% with 90 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.69%. Comparing base (a46f02c) to head (64e1112).
⚠️ Report is 12 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/openai/src/client.rs	0.00%	36 Missing ⚠️
crates/openai/src/types.rs	25.00%	30 Missing ⚠️
crates/anthropic/src/client/messages.rs	0.00%	14 Missing ⚠️
src/combo/starter.rs	20.00%	4 Missing ⚠️
src/provider/openai.rs	20.00%	4 Missing ⚠️
src/cmd/prompt.rs	66.66%	1 Missing ⚠️
src/combo/session.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #156      +/-   ##
==========================================
- Coverage   50.22%   47.69%   -2.54%     
==========================================
  Files          93      100       +7     
  Lines       25033    27252    +2219     
==========================================
+ Hits        12574    12999     +425     
- Misses      12459    14253    +1794     

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