fix: make usage chunk in stream mode of gemini compatible with openai

[hustxiayang]

Description

Users found the simply use "usage" information does not work for streaming responses of gemini models.

This is because for openai models, the usage chunk would be a separate chunk. For example, this is an example response from gpt-4o:

...

chunk=ChatCompletionChunk(id='chatcmpl-CYv6DPkWfT1xrsS2ySOoRztQKnZDg', choices=[Choice(delta=ChoiceDelta(content=None, function_call=None, refusal=None, role=None, tool_calls=None), finish_reason='stop', index=0, logprobs=None, content_filter_result={'error': {'code': 'content_filter_error', 'message': 'The contents are not filtered'}})], created=1762438677, model='azure.gpt-4o', object='chat.completion.chunk', service_tier=None, system_fingerprint='fp_4a331a0222', usage=None, obfuscation='2xP')


chunk=ChatCompletionChunk(id='chatcmpl-CYv6DPkWfT1xrsS2ySOoRztQKnZDg', choices=[], created=1762438677, model='azure.gpt-4o', object='chat.completion.chunk', service_tier=None, system_fingerprint='fp_4a331a0222', usage=CompletionUsage(completion_tokens=17, prompt_tokens=12, total_tokens=29, completion_tokens_details=CompletionTokensDetails(accepted_prediction_tokens=0, audio_tokens=0, reasoning_tokens=0, rejected_prediction_tokens=0), prompt_tokens_details=PromptTokensDetails(audio_tokens=0, cached_tokens=0)), obfuscation='xY40HsJr')

There is a finish_reason chunk, and then a usage chunk.

Thus, want to make it compatible with Openai. (Actually, in anthropic translation, it's already compatible)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 61.90476% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.01%. Comparing base (96a2fdd) to head (30a3391).

Files with missing lines	Patch %	Lines
internal/translator/openai_gcpvertexai.go	61.90%	4 Missing and 4 partials ⚠️

❌ Your patch status has failed because the patch coverage (61.90%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1503      +/-   ##
==========================================
- Coverage   84.03%   84.01%   -0.02%     
==========================================
  Files         141      141              
  Lines       13039    13049      +10     
==========================================
+ Hits        10957    10963       +6     
- Misses       1460     1462       +2     
- Partials      622      624       +2     

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[hustxiayang]

/retest

[nacx]

@yuzisun can you take a look at this one?