Update /json endpoint default model and Workers AI auth docs

[kathayl]

• Update default model from @cf/meta/llama-3.3-70b-instruct-fp8-fast to @cf/google/gemma-4-26b-a4b-it
• Add note that authorization is not required for Workers AI models in custom_ai
• Update fallback example to use new default model and drop authorization for Workers AI entry

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Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[github-actions]

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

Pattern	Owners
/src/content/docs/browser-rendering/	@mchenco, @cloudflare/pcx-technical-writing, @celso, @kathayl, @ToriLindsay

[github-actions]

Preview URL: https://9bae6d9e.preview.developers.cloudflare.com
Preview Branch URL: https://br-json-default-model-update.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/browser-rendering/rest-api/json-endpoint/	https://br-json-default-model-update.preview.developers.cloudflare.com/browser-rendering/rest-api/json-endpoint/

[kathayl]

@copilot resolve the merge conflicts in this pull request

[kathayl]

note: we reverted back to old default model so holding this PR until we made the switch back to gemma

[marciocloudflare]

PCX triage note — PCX team contacted the author on 2026-04-15; author replied on 2026-04-15 — waiting for a solution.

[github-actions]

Hey there, we've marked this pull request as stale because there's no recent activity on it. This label helps us identify PRs that might need updates (or to be closed out by our team if no longer relevant).

[github-actions]

Hey there, we've closed out this pull request because it's been stale for a while and there's been no additional action on it. If these changes are still relevant, open a new pull request (or flag to us in a GitHub issue).