Fix Vertex AI tool response roles

[Gujiassh]

Summary

• normalize Gemini-style tool response messages to role: 'user' when Vertex AI builds generateContent payloads
• keep the fix scoped to the Vertex AI chat wrapper without changing Gemini's native payload shape
• add a focused provider spec proving function-response payloads use the Vertex AI-compatible role while preserving the existing function response structure

Validation

• bundle exec rspec spec/ruby_llm/providers/vertex_ai_spec.rb
• bundle exec rubocop lib/ruby_llm/providers/vertexai/chat.rb spec/ruby_llm/providers/vertex_ai_spec.rb