feat: Add Web search tool for OpenAI Client

[majiayu000]

Fixes #1333

Changes

• Add WebSearchOptions, UserLocation, and ApproximateLocation types to support OpenAI web search
• Add WithWebSearch call option for enabling web search on supported models (gpt-4o-search-preview)
• Implement conversion from llms types to OpenAI client types
• Add comprehensive tests for web search options marshaling and conversion

Usage

llm.GenerateContent(ctx, messages,
    llms.WithWebSearch(&llms.WebSearchOptions{
        SearchContextSize: "high",
        UserLocation: &llms.UserLocation{
            Type: "approximate",
            Approximate: &llms.ApproximateLocation{
                Country: "US",
                City:    "San Francisco",
            },
        },
    }),
)