feat: implement exponential backoff for retry logic

[linw1995]

Summary

This PR implements exponential backoff retry mechanism for provider API calls (Anthropic and OpenAI), addressing issue #114.

Changes

Core Implementation

• New retry module (src/retry.rs): Defines RetryAttempt, RetryUpdate, and RetryNotifier types for retry event tracking
• Configuration: Added retry_max_attempts (default: 3) and retry_max_delay_ms (default: 60000ms) to RequestOptions and ModelRequestConfig
• Retry logic: Uses the backon crate with ExponentialBuilder including jitter to prevent thundering herd problem

Provider API Clients

• OpenAI client (crates/openai/src/client.rs):

  • Wrapped all API calls with exponential backoff retry
  • Added OpenAIRequestError with proper error classification (transport, HTTP status, decode)
  • Retryable status codes: 429 (Too Many Requests), 500 (Internal Server Error), 503 (Service Unavailable)
  • Network timeouts and connection errors are also retryable
  • Non-retryable: 400, 401, 403, 404, and decode errors

• Anthropic client (crates/anthropic/src/client/messages.rs):

  • Same retry pattern applied to both streaming and non-streaming message APIs
  • Consistent error classification with OpenAI implementation

UI Integration

• TUI chat component (crates/coco-tui/src/components/chat.rs):
  • Added retry status indicator showing current retry attempt and delay before next attempt
  • Displays format: "retrying in {delay} (attempt {current}/{max})"
  • Status clears when retry finishes (success or final failure)

Logging & Observability

• Each retry attempt logs the error and delay duration using tracing::warn
• Retry events are propagated through RetryNotifier callback system
• Integrates with TUI event system for real-time status updates

Issue Reference

Resolves #114

[codecov-commenter]

Codecov Report

❌ Patch coverage is 9.77131% with 434 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.22%. Comparing base (e5977ab) to head (3be0148).
⚠️ Report is 20 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/anthropic/src/client/messages.rs	6.36%	147 Missing ⚠️
crates/openai/src/client.rs	6.57%	142 Missing ⚠️
src/provider.rs	0.00%	39 Missing ⚠️
crates/anthropic/src/client.rs	0.00%	35 Missing ⚠️
crates/coco-tui/src/components/chat.rs	0.00%	30 Missing ⚠️
src/provider/openai.rs	0.00%	25 Missing ⚠️
src/retry.rs	0.00%	11 Missing ⚠️
src/agent.rs	40.00%	3 Missing ⚠️
src/config/provider.rs	92.59%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #123      +/-   ##
==========================================
- Coverage   53.46%   50.22%   -3.25%     
==========================================
  Files          88       93       +5     
  Lines       21248    25033    +3785     
==========================================
+ Hits        11360    12572    +1212     
- Misses       9888    12461    +2573     

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