fix: resolve Sandbox proxy 502 in Cloudflare Workers

[sam2tom]

Sandbox Playground requests fail in Cloudflare Workers before reaching the upstream because Workers rejects redirect: "error". The proxy catches that TypeError and returns 502 request_failed_result_unknown, even though direct v2 template discovery succeeds.

Use redirect: "manual" and explicitly reject every 3xx response on both the sandbox access check and final upstream request. Return a safe 502 without forwarding Location, cookies, or upstream bodies; credentials never follow a redirect.

Validation:

• Actual proxy source reproduced the exact 502 in local workerd; the fixed source returned 200 with a synthetic template response. Local runtime compatibility date: 2026-08-06; deployment configuration is unchanged.
• 14 Sandbox tests and Worker routing tests passed, including 301/302/303/307/308 responses at the control, access-check, and data stages.
• Read-only direct production v2 discovery returned 200 with four templates. The terminal's request to the public docs route returned a separate non-JSON 403, so a deployed end-to-end success is not yet claimed.

No deployment or Sandbox resource mutations performed.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-29T06:33:35.949641Z	23af56c	PR opened

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