translator: support field cache_write_tokens for openai

[aabchoo]

Description

Adding an additional field cache_write_tokens for openai translation and marking cache_creation_tokens as deprecated in v1.3.0. This fixes cache writes not being accounted for for gpt-5.6+ models.

Openai translated fields returns cache_write_tokens as part of the usage response:

  "usage": {
    "prompt_tokens": 1116,
    "completion_tokens": 278,
    "total_tokens": 1394,
    "prompt_tokens_details": {
      "cached_tokens": 0,
      "cache_creation_tokens": 1113, # same
      "cache_write_tokens": 1113,
    },
}

https://developers.openai.com/api/docs/guides/prompt-caching/diagnostics#confirm-the-improvement

Updating the translation layer to parse the cache_write_tokens from/to openai as well. Keeping the legacy field in temporarily but will move to remove it in a later release. Cache write tokens created by openai are currently ignored.

This means the response of openai translation will be cache_write_tokens in addition to cache_creation_tokens.

Related Issues/PRs (if applicable)

n/a

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[codecov]

Codecov Report

❌ Patch coverage is 89.58333% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/translator/openai_completions.go	50.00%	2 Missing ⚠️
internal/translator/openai_openai.go	50.00%	2 Missing ⚠️
internal/translator/openai_awsbedrock.go	50.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[sukumargaonkar]

i understand we are renaming because openai recently added the cache-write field
but since this will remove a field from the response users receives, i think we should do the field-deletion over atleast 2 minor releases
for this release we can have both CacheCreationTokens and CacheWriteTokens fields in response, note CacheWriteTokens as deprecated in upcoming release and remove it 2 versions down the line