Add Gemini 3.1 Pro customtools model to Vertex AI provider

[NVolcz]

Inspired by #11608.

Related GitHub Issue

Roo Code Task Context (Optional)

Description

Add gemini-3.1-pro-preview-customtool to the Vertex AI provider.

Test Procedure

• Type check: pnpm --filter @roo-code/types check-types
• - Provider tests: pnpm --filter roo-cline exec vitest run api/providers/__tests__/gemini.spec.ts api/providers/__tests__/vertex.spec.ts
• Manual testing

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes (if applicable).
• Documentation Impact: I have considered if my changes require documentation updates (see "Documentation Updates" section below).
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Screenshots / Videos

N/A (no UI changes)

Documentation Updates

• No documentation updates are required.
• Yes, documentation updates are required. (Please describe what needs to be updated or link to a PR in the docs repository).

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