feat(go): added iterator-based streaming functions and typed prompts

[apascal07]

This PR adds Go iterator-based streaming support for model generation and prompt execution, enabling real-time consumption of model output chunks.

New Features

1. GenerateStream / GenerateDataStream - Stream model responses using Go's range-over-func pattern:

type StoryOutput struct { Title, Content string }

for result, err := range genkit.GenerateDataStream[StoryOutput, StoryOutput](ctx, g,
    ai.WithModelName("googleai/gemini-2.5-flash"),
    ai.WithPrompt("Tell me a story"),
) {
    if err != nil {
        return err
    }
    if result.Done {
        fmt.Println("Final:", result.Output.Title)
    } else {
        fmt.Print(result.Chunk.Content) // Stream chunks as they arrive
    }
}

2. Prompt.ExecuteStream - Stream prompt execution:

prompt := genkit.LookupPrompt(g, "myPrompt")
for result, err := range prompt.ExecuteStream(ctx, ai.WithInput(input)) {
    if err != nil {
        return err
    }
    if !result.Done {
        cb(ctx, result.Chunk) // Forward chunks to flow callback
    }
}

3. DataPrompt with typed streaming - Strongly-typed prompts with streaming:

type StoryInput struct { Topic string `json:"topic"` }
type StoryOutput struct { Title, Content string }

storyPrompt := genkit.DefineDataPrompt[StoryInput, StoryOutput, StoryOutput](
    g, "story",
    ai.WithModelName("googleai/gemini-2.5-flash"),
    ai.WithPrompt("Write a story about {{topic}}"),
)

// Alternatively, look up and wrap a .prompt file:
// storyPrompt := genkit.LookupDataPrompt[StoryInput, StoryOutput, StoryOutput](g, "story")

for result, err := range storyPrompt.ExecuteStream(ctx, StoryInput{Topic: "dragons"}) {
    if err != nil {
        return err
    }
    if result.Done {
        fmt.Printf("Title: %s\n", result.Output.Title)
    }
}