[AI Search] Add changelog for namespace Wrangler commands

[chuetten25]

Summary

Adds a changelog entry announcing namespace-level Wrangler commands for AI Search (wrangler ai-search namespace list/create/get/update/delete), plus the new --namespace flag on instance-level commands. Documents --json output support for automation and agent workflows, and links to the AI Search Wrangler commands documentation.

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.

[cloudflare-docs-bot]

Review

✅ No style-guide issues found in commit db87145.

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[aninibread]

Can you also add a release note inside AI Search?

it should be under src/content/release-notes/ai-search.yaml