docs: Update image understanding model

[anirudh161]

Update model used in image generation code sample

Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Please merge this PR for me once it is approved

[gemini-code-assist]

Summary of Changes

Hello @anirudh161, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on a minor but important update to a Node.js code sample, specifically upgrading the version of the Gemini model used for image understanding to leverage the latest capabilities.

Highlights

• Model Update: The gemini-2.0-flash model has been updated to gemini-2.5-flash within the textgen-with-txt-img.js sample.

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[gemini-code-assist]

Code Review

This pull request updates the model name for the image understanding sample. The change is straightforward and correct. I have one suggestion to improve the code's maintainability by defining the model name as a constant, which is a good practice for configuration values, especially in sample code that others may build upon.

[gemini-code-assist]

For improved maintainability and readability, it's a good practice to declare string literals for configuration, like model names, as constants. This avoids using 'magic strings' directly in the code.

By defining it as a constant at the top of the file, it becomes easier to locate and update. This is particularly helpful for values that might change, such as model versions.

Example:

const MODEL_NAME = 'gemini-2.5-flash';

// ... later in the code
const response = await ai.models.generateContent({
  model: MODEL_NAME,
  // ...
});