Linked Issue

Closes #3942

Description

Google deprecated text-embedding-004 in January 2026. This PR updates the default embedding model to gemini-embedding-001 across both the Gemini and Vertex AI embedders, along with their docs and
tests.

Changes:

• mem0/embeddings/gemini.py — default model models/text-embedding-004 → models/gemini-embedding-001
• mem0/embeddings/vertexai.py — default model text-embedding-004 → gemini-embedding-001
• Updated docs for both google_AI.mdx and vertexai.mdx (examples + config tables)
• Added tests verifying default model and embedding dims for both embedders
• Updated existing Vertex AI test references from text-embedding-004 to gemini-embedding-001

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (no functional changes)
• Documentation update

Breaking Changes

N/A

Test Coverage

• I added/updated unit tests
• I added/updated integration tests
• I tested manually (describe below)
• No tests needed (explain why)

All 13 embedding tests pass (test_gemini_emeddings.py + test_vertexai_embeddings.py).

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have added tests that prove my fix/feature works
• New and existing tests pass locally
• I have updated documentation if needed