feat(go/plugins/vertexai): Model Garden support for Claude 4.6 and 4.7

[cabljac]

Summary

• Registers three Claude models in the Vertex Model Garden plugin:

  • claude-sonnet-4-6 (Claude 4.6 Sonnet)
  • claude-opus-4-6 (Claude 4.6 Opus)
  • claude-opus-4-7 (Claude 4.7 Opus)

• Adds matching demo flows in go/samples/modelgarden/main.go:

  • claudeSonnet46VertexModelGardenFlow
  • claudeOpus46VertexModelGardenFlow
  • claudeOpus47VertexModelGardenFlow

• Each sample flow passes MaxTokens in the Anthropic request config so Model Garden requests are valid.

Test plan

• gofmt on changed Go files
• go test ./go/plugins/vertexai/modelgarden/...
• go build ./go/plugins/vertexai/modelgarden/... ./go/samples/modelgarden/...
• go vet clean
• Dev UI reflection verified for:
  • /model/vertexai/claude-sonnet-4-6
  • /model/vertexai/claude-opus-4-6
  • /model/vertexai/claude-opus-4-7
  • /flow/claudeSonnet46VertexModelGardenFlow
  • /flow/claudeOpus46VertexModelGardenFlow
  • /flow/claudeOpus47VertexModelGardenFlow
• Manual flow execution: Sonnet 4.6 returns text successfully.

Checklist

• PR title follows Conventional Commits
• Tested (package tests + manual Dev UI run)
• Docs updated (not required for this code-only change)

Closes #136
Closes #137

[gemini-code-assist]

Code Review

This pull request adds support for Claude Sonnet 4.6 and Claude Opus 4.6 models to the Vertex AI Model Garden plugin and includes sample flows demonstrating their usage. Feedback was provided regarding the naming consistency of the model labels to ensure they align with existing patterns in the codebase.