feat(ai-sandbox-vercel): tag sandbox API requests with a framework user-agent

[marc-vercel]

🎯 Changes

On the Vercel side, we would like to be able to distinguish the requests that come from @tanstack/ai.

This PR adds the package name @tanstack/ai as a token in the User-Agent HTTP header for all Vercel Sandbox API requests (create, resume, destroy). Existing user-agent values are preserved and the token is appended rather than replacing them. We have already done something similar with the Eve framework: vercel/eve#332.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

Testing notes: unit tests were added in packages/ai-sandbox-vercel/tests/user-agent.test.ts covering token appending/defaulting, header/field preservation, Request input handling, and fallback fetch.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Vercel Sandbox requests now include an added @tanstack/ai token in the user-agent header.
  • Existing user-agent values are preserved and updated rather than replaced.

• Tests

  • Added coverage for header handling, request forwarding, and default fetch behavior.

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a withSandboxUserAgent fetch wrapper in the Vercel sandbox package that appends an @tanstack/ai token to the user-agent header. VercelProvider now uses this wrapped fetch in Sandbox.create, resume, and destroy. Includes tests and a changeset.

Changes

Vercel sandbox user-agent tagging

Layer / File(s)	Summary
User-agent fetch wrapper packages/ai-sandbox-vercel/src/user-agent.ts	Adds USER_AGENT_TOKEN constant and withSandboxUserAgent function that wraps fetch to set or append the token to the user-agent header, forwarding to an inner fetch or globalThis.fetch.
Provider wiring packages/ai-sandbox-vercel/src/provider.ts	VercelProvider imports withSandboxUserAgent, adds a private fetch field, and passes fetch: this.fetch into Sandbox.create, and Sandbox.get calls in resume and destroy.
Tests and changeset packages/ai-sandbox-vercel/tests/user-agent.test.ts, .changeset/vercel-sandbox-user-agent.md	Adds a Vitest suite covering token appending/defaulting, header/field preservation, Request input handling, and fallback fetch; adds a changeset documenting the patch.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: AlemTuzlak

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly matches the main change: adding a User-Agent token to ai-sandbox-vercel requests.
Description check	✅ Passed	The description follows the template and includes changes, checklist items, testing notes, and release impact.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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