[FEATURE] Add VERTEX_REGION_CLAUDE_4_5_SONNET

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

We'd like to be able to specify if we use Sonnet 4.5 in EU or US

Proposed Solution

Add VERTEX_REGION_CLAUDE_4_5_SONNET just like we have for others:

# Optional: Override regions for other specific models
export VERTEX_REGION_CLAUDE_3_5_SONNET=us-east5
export VERTEX_REGION_CLAUDE_3_7_SONNET=us-east5
export VERTEX_REGION_CLAUDE_4_0_OPUS=europe-west1
export VERTEX_REGION_CLAUDE_4_0_SONNET=us-east5
export VERTEX_REGION_CLAUDE_4_1_OPUS=europe-west1

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

_No response_