feat(chat_completions): support partial mode

[northes]

Describe the change

Support partial mode.

Provide Moonshot documentation link

https://platform.moonshot.cn/docs/api/partial#partial-mode

Describe your solution

Add Partial and Name fields to ChatCompletionsMessage struct and test code.

Tests

Add TestPartialMode method in api_chat_completions_test.go, pass.

Additional context

Nothing.

Issue: #14

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 71.08%. Comparing base (12b8194) to head (62e528b).

Files	Patch %	Lines
api_chat_completions.go	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   71.03%   71.08%   +0.04%     
==========================================
  Files          11       11              
  Lines         366      377      +11     
==========================================
+ Hits          260      268       +8     
- Misses         58       60       +2     
- Partials       48       49       +1     

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