fix: provider API retry with exponential backoff

[linw1995]

Implements automatic retry mechanism for Provider API calls with exponential backoff strategy.

• Retries on transient network errors and decoding errors
• Uses exponential backoff between retries
• Provides clear error messages after all retries exhausted

Fixes #133

[codecov-commenter]

Codecov Report

❌ Patch coverage is 13.70192% with 359 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.06%. Comparing base (a46f02c) to head (372a1dc).
⚠️ Report is 2 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/agent.rs	5.79%	130 Missing ⚠️
src/tools/run_combo.rs	0.00%	66 Missing ⚠️
crates/coco-tui/src/components/chat.rs	0.00%	57 Missing ⚠️
crates/openai/src/client.rs	22.85%	27 Missing ⚠️
crates/anthropic/src/client/messages.rs	55.31%	21 Missing ⚠️
src/tools/run_task.rs	0.00%	16 Missing ⚠️
src/error.rs	0.00%	12 Missing ⚠️
src/provider.rs	0.00%	9 Missing ⚠️
src/stream_error.rs	62.50%	9 Missing ⚠️
src/provider/openai.rs	0.00%	7 Missing ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #137      +/-   ##
==========================================
- Coverage   50.22%   49.06%   -1.17%     
==========================================
  Files          93       94       +1     
  Lines       25033    25715     +682     
==========================================
+ Hits        12574    12617      +43     
- Misses      12459    13098     +639     

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