[Workers AI] Use a newer model in the Wrangler starter guide

[superhighfives]

Summary

Updates the Workers AI Wrangler starter guide to use
@cf/google/gemma-4-26b-a4b-it, a newer model generation. The example now
uses Gemma's documented messages input, disables reasoning for the basic
request, and shows the chat-completion response shape.

Fixes #32501

PR #32728 proposes the still-active
@cf/meta/llama-3.1-8b-instruct-fast variant. The original version of this
PR used @cf/meta/llama-3.2-3b-instruct; this update supersedes both
alternatives by using Gemma 4.

Validation

• Astro and Worker checks passed
• ESLint and formatting checks passed
• Node, Astro, and Workers tests passed
• Full documentation build passed
• Live Gemma 4 smoke test unavailable because Wrangler is not authenticated

Documentation checklist

• The change adheres to the documentation style guide.
• An issue has been opened for the outdated information this PR fixes.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/workers-ai/	@rita3ko, @craigsdennis, @mchenco, @zeke, @superhighfives, @shridhar-cf, @mattrothenberg, @ethulia, @aninibread, @kflansburg, @cloudflare/product-owners

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Original Link	Updated Link
https://developers.cloudflare.com/workers-ai/get-started/workers-wrangler/	https://fix-workers-ai-newer-starter-model.preview.developers.cloudflare.com/workers-ai/get-started/workers-wrangler/

[cloudflare-docs-bot]

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