Fix: remove unsupported prompt-caching-scope-2026-01-05 header for vertex ai

[Sameerlite]

Relevant issues

Fixes #19984

Pre-Submission checklist

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

CI (LiteLLM team)

CI status guideline:

• 50-55 passing tests: main is stable with minor issues.
• 45-49 passing tests: acceptable but needs attention
• <= 40 passing tests: unstable; be careful with your merges and assess the risk.

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Type

🆕 New Feature
🐛 Bug Fix
🧹 Refactoring
📖 Documentation
🚄 Infrastructure
✅ Test

Changes

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Project	Deployment	Actions	Updated (UTC)
litellm	Error		Jan 30, 2026 0:06am