Add cost tracking for streaming in vertex ai passthrough

[Sameerlite]

Title

Add cost tracking for streaming in vertex ai

Relevant issues

Pre-Submission checklist

Please complete all items before asking a LiteLLM maintainer to review your PR

• [x] I have Added testing in the tests/litellm/ directory, Adding at least 1 test is a hard requirement - see details
• [x] I have added a screenshot of my new test passing locally
• [x] My PR passes all unit tests on make test-unit
• [x] My PR's scope is as isolated as possible, it only solves 1 specific problem

Type

🆕 New Feature 🐛 Bug Fix

Changes

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Project	Deployment	Preview	Comments	Updated (UTC)
litellm	Ready	Preview	Comment	Nov 26, 2025 4:19pm

[ishaan-jaff]

rebase to main

[CLAassistant]

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