Rename Vertex AI to Gemini Enterprise Agent Platform in Google client

[markuswustenberg]

Once that's available.

See https://docs.cloud.google.com/gemini-enterprise-agent-platform/release-notes

[maragubot]

Checked against google.golang.org/genai v1.54.0 (latest as of 2026-04-23, and the version currently pinned in go.mod): still exposes BackendVertexAI and BackendGeminiAPI, with no AgentPlatform / "Gemini Enterprise" identifiers anywhere in the package. Nothing to do here until the upstream SDK renames.