Fixed bug usage.prompt_tokens_details=None

[NikkiAung]

• I understand that this repository is auto-generated and my pull request may not be merged

Changes being requested

Problem

When Gemini API responses omitted prompt_tokens_details or completion_tokens_details from the usage object, the OpenAI Python library was setting them to None instead of objects with None values:

'prompt_tokens_details': None # ❗️Wrong

Solution

Added preprocessing in src/openai/_models.py to ensure missing fields are initialized as empty dicts before Pydantic validation:
Created _preprocess_completion_usage() function

• Detects CompletionUsage type
• Ensures prompt_tokens_details and completion_tokens_details are {} if missing/None
• Empty dicts cause Pydantic to construct nested models with default None values

'prompt_tokens_details': {'audio_tokens': None, 'cached_tokens': None} # ✅ Correct

Additional context & links

After code changes, you can see audio_tokens and cached_tokens become available in None instead of "prompt_tokens_details": None below demo.

6B30DF76-43F3-4148-81E9-718E4672065F.MOV

To test out this changes, use this file

Fixes #2544