fix(cost): align vertex_ai/gemini-embedding-2 GA source URL (followup #27848)

[mateo-berri]

Address Greptile P2 concern on #27848: GA vertex_ai/gemini-embedding-2 source URL still pointed to ai.google.dev while the preview entry was updated to the Vertex AI pricing page. Both share identical pricing values; sync source URL for consistency.

https://claude.ai/code/session_01W8jRwstnmduadGw8Z8egxe

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Note

Low Risk
Low risk metadata-only change: updates the source URL for vertex_ai/gemini-embedding-2 pricing without altering any costs or limits.

Overview
Updates the source link for the vertex_ai/gemini-embedding-2 embedding model in both model_prices_and_context_window.json and litellm/model_prices_and_context_window_backup.json, pointing from the Gemini embeddings doc to the Vertex AI pricing page for consistency.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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