[AIT-205] Add Vercel AI SDK message per token guide

[matt423]

Description

Follows the structure and content of other message-per-token guides for the Vercel AI SDK.

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Checklist

• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

Summary by CodeRabbit

• Documentation
  • Added new guide for streaming Vercel AI SDK responses via Ably using the message-per-token pattern
  • Includes publisher and subscriber workflow examples with step-by-step setup and execution instructions
  • Covers multi-publisher and multi-subscriber scenarios with per-response correlation

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Walkthrough

This PR adds comprehensive documentation for implementing token streaming with the Vercel AI SDK using Ably's message-per-token pattern. It includes navigation updates, a guide tile, and a detailed guide with publisher and subscriber code examples demonstrating real-time response reconstruction.

Changes

Cohort / File(s)	Summary
Navigation Entry src/data/nav/aitransport.ts	Added new navigation item "Vercel AI SDK token streaming - message per token" under Guides, linking to the new documentation page.
Documentation Index src/pages/docs/ai-transport/index.mdx	Introduced new "Vercel AI SDK" subsection with a Tile component linking to the vercel-message-per-token guide with title, description, and image.
New Guide Documentation src/pages/docs/guides/ai-transport/vercel-message-per-token.mdx	Added comprehensive guide covering the message-per-token streaming pattern, including Vercel SDK event mapping to Ably messages, publisher workflow with streamText iteration, subscriber workflow with response reconstruction per responseId, multi-publisher/subscriber scenarios, event sequences, and setup guidance.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

🐰 Hops through streams of tokens bright,
Vercel's SDK shines with Ably's light,
Messages flow per token cascade,
Publishers and subscribers promenade,
Real-time responses, granular and keen,
The finest streaming guide you've seen! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly corresponds to the main change: adding documentation for Vercel AI SDK message-per-token streaming guidance as specified in issue AIT-205.
Linked Issues check	✅ Passed	The PR successfully delivers the objective from AIT-205: a guide article demonstrating token streaming with Vercel AI SDK, including publisher/subscriber patterns and granular history management.
Out of Scope Changes check	✅ Passed	All changes are within scope: adding a new navigation entry, index tile reference, and comprehensive guide documentation specifically for Vercel AI SDK message-per-token streaming as required by AIT-205.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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[matt423]

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[mschristensen]

Thanks, looks good, just a few small comments but approving ahead of those changes.

[mschristensen]

Should this be:

Suggested change

	Export your AI provider API key to the environment, which will be used later in the guide by the Vercel AI SDK:
	Export your Vercel AI Gateway API key to the environment, which will be used later in the guide by the Vercel AI SDK:

I'm looking at: https://ai-sdk.dev/docs/getting-started/nodejs#configure-vercel-ai-gateway-api-key