docs(vertex): show image provider setup

[Kage18]

Summary

• update the Google Vertex image-generation example to show explicit provider setup
• include project and location before calling googleVertex.image

Fixes #7587.

Kage usage

• Used Kage scoped to packages/google-vertex to capture the docs nuance for future agents.
• Local Kage metrics after refresh: 69 files, 751 symbols, 784 calls, 204 tests, 100% indexed coverage, 3 memory packets, 48 memory graph entities, 50 evidence-backed edges, 0 stale packets.

Tests

• git diff --check

Docs-only change.

[Kage18]

Kage usage for this PR:

This PR fixed #7587 by showing explicit Google Vertex image-provider setup with project and location before calling googleVertex.image(...).

How Kage helped:

• Scoped recall to the Google Vertex provider docs and package context.
• Identified the exact target doc: content/providers/01-ai-sdk-providers/16-google-vertex.mdx.
• Helped keep the fix focused on setup clarity rather than changing provider code.
• Captured the reusable docs nuance: Vertex image examples should show provider configuration explicitly because project/location are required context for many users.
• Captured the docs-only verification path: git diff --check.

Why this matters for future sessions:
A future agent editing Vertex docs can recall that image-generation examples should include concrete provider setup, not just the final model call.

Full contribution summary and Kage metrics: kage-core/Kage#8