feat: add token usage tracking and display

[linw1995]

No description provided.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 25.68306% with 136 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.94%. Comparing base (e5977ab) to head (7c4140f).
⚠️ Report is 5 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/coco-tui/src/components/chat.rs	0.00%	59 Missing ⚠️
src/agent.rs	29.54%	31 Missing ⚠️
src/provider/openai.rs	0.00%	23 Missing ⚠️
src/provider/anthropic.rs	0.00%	18 Missing ⚠️
crates/openai/src/client.rs	0.00%	3 Missing ⚠️
src/provider/types.rs	94.28%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #102      +/-   ##
==========================================
- Coverage   53.46%   50.94%   -2.52%     
==========================================
  Files          88       91       +3     
  Lines       21248    23317    +2069     
==========================================
+ Hits        11360    11880     +520     
- Misses       9888    11437    +1549     

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