feat(ts-sdk): add Vertex AI embedding provider

Summary

The Python SDK supports Vertex AI as an embedding provider, but the TypeScript OSS SDK (mem0ai/oss) does not. Add it to bring the TS SDK to parity.

Python reference	mem0/embeddings/vertexai.py
Registered in (Python)	mem0/utils/factory.py (EmbedderFactory)
Target file (TypeScript)	mem0-ts/src/oss/src/embeddings/vertexai.ts
Suggested implementation	Use @google-cloud/aiplatform (PredictionServiceClient) text-embedding models.

Requirements

• Implement VertexAIEmbedder in mem0-ts/src/oss/src/embeddings/vertexai.ts, extending Embedder (mem0-ts/src/oss/src/embeddings/base.ts) and mirroring the Python provider's behavior (embed / embedBatch).
• Register the "vertexai" provider in mem0-ts/src/oss/src/utils/factory.ts (EmbedderFactory).
• Add config typing in mem0-ts/src/oss/src/types/.
• Add a unit test under mem0-ts/src/oss/src/tests/.
• Add @google-cloud/aiplatform to mem0-ts/package.json (optional/peer dependency, lazy-imported like other providers).
• Update docs under docs/ if this provider is user-facing.

Reference pattern

Mirror an existing TS provider: embeddings/openai.ts.

Notes

GCP auth (ADC / service account). Mirror mem0/embeddings/vertexai.py.

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Part of the TypeScript ↔ Python SDK provider-parity effort. One provider per issue (atomic).