Feature 5 — Implement Google Vertex AI Provider

**Outcome:**

* Add `VertexAiChatModel` via LangChain4j.
* Support Service Account JSON, Project ID, Region.
* E2E test with Gemini-Pro or embeddings.
* Add provider-specific config fields to the UI.

**Why:**
- Covers final major cloud for regulated industries.
- "dotCMS is truly cloud-agnostic for AI."

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**Related Epic:** #32634