bugfix: include reasoning tokens to completion tokens for google provider

[modestaspruckus]

Background

Gemini 2.5 pro latest snapshot 06-05 have thinking mode enabled by default. It is not reliable to track costs of tokens because reasoning tokens are not accumulated into output tokens.

Summary

• I have added additional method which calculates reasoning tokens from total token count and accumulates them into output tokens.

Verification

• usageMetadata returns totalTokenCount where totalTokenCount != (promptTokenCount + candidatesTokenCount) because thoughtsTokenCount is not included

Tasks

• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• Formatting issues have been fixed (run pnpm prettier-fix in the project root)

Future Work

Ideally sdk could expose thoughtTokens seperately or proxy total token count without auto calculation.

Related Issues

#6082

[modestaspruckus]

@lgrammel could you take a look at this one pls?