docs(dart/models): use new top-level system parameter for system prompts

[chrisraygill]

Summary

Bug fix. The Dart variant of the Models / generating content docs previously showed config: GeminiOptions(systemInstruction: ...) as the way to set a system prompt. That is a Gemini-specific config option, not the framework-level way to specify a system prompt — it doesn't work with non-Gemini models and bypasses Genkit's role-based message system. The snippet was incorrect for a section meant to document the cross-model ai.generate() API.

This PR replaces it with the new top-level system parameter on ai.generate(), which matches the JS and Go SDK surface.

Depends on

genkit-ai/genkit-dart#289 — adds the system parameter to Genkit.generate, Genkit.generateStream, lite.generate, lite.generateStream, and generateHelper. This docs change should land after that PR ships in a release that the docs target.

Before (incorrect)

final response = await ai.generate(
  model: googleAI.gemini('gemini-2.5-flash'),
  prompt: 'Invent a menu item for a pirate themed restaurant.',
  config: GeminiOptions(
    systemInstruction: 'You are a food industry marketing consultant.',
  ),
);

After

final response = await ai.generate(
  model: googleAI.gemini('gemini-2.5-flash'),
  system: 'You are a food industry marketing consultant.',
  prompt: 'Invent a menu item for a pirate themed restaurant.',
);

Test plan

• pnpm generate-language-pages regenerates src/content/docs/docs/dart/models.mdx with the new snippet (verified locally; that path is gitignored).
• Reviewer checks the rendered Dart Models page looks right.

[gemini-code-assist]

Code Review

This pull request updates the Dart documentation in models.mdx to show how to provide a system prompt using the direct system parameter instead of configuring it through GeminiOptions. There are no review comments to address, and the changes look correct.