fix(text-generation): update Gemini model version

[laxmi-genai]

Description

Update to gemini-2.5-flash from gemini-2.0-flash. This file is referenced in https://cloud.google.com/vertex-ai/generative-ai/docs/start/quickstart?usertype=adc#googlegenaisdk_textgen_with_txt-nodejs_genai_sdk and the Python SDK example refers to gemini-2.5-flash. Submitting this PR so that the NodeJS code has the same version

Fixes #

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Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• Required CI tests pass (see CI testing)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

Note: Any check with (dev), (experimental), or (legacy) can be ignored and should not block your PR from merging (see CI testing).

[gemini-code-assist]

Summary of Changes

Hello @laxmi-genai, 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 updating the Gemini model version used in a specific Node.js text generation sample. The core intent is to maintain consistency with the latest documentation and other SDK examples, providing users with an accurate and up-to-date quickstart experience when interacting with the Gemini API.

Highlights

• Model Version Update: The gemini-2.0-flash model has been updated to gemini-2.5-flash in the textgen-with-txt.js sample.
• Documentation Alignment: This change ensures the Node.js sample aligns with the model version referenced in the official Vertex AI Generative AI documentation and the Python SDK examples, improving consistency for users.

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[gemini-code-assist]

Code Review

This pull request correctly updates the Gemini model version from gemini-2.0-flash to gemini-2.5-flash in the NodeJS text generation sample. The change is straightforward and aligns with the goal stated in the description. I have provided one suggestion to improve maintainability by extracting the hardcoded model name into a constant, which is a common best practice.