Hunyuan3D OFFICIAL_API: SignatureFailure persists after restart with AI3D 3.0 addon

Summary

After the AI3D 3.0 update from #294, OFFICIAL_API still returns AuthFailure.SignatureFailure on a fresh Blender MCP restart.

Environment

• macOS (Apple Silicon)
• Blender 5.2.1 LTS
• Blender MCP addon reports version 1.6, protocol 5
• Installed from the current main checkout (commit 9224fe3)
• Hunyuan3D enabled; mode: OFFICIAL_API
• Tencent Cloud CAM SecretId/SecretKey supplied (both redacted; SecretId begins with AKID)

Reproduction

1. Restart Blender MCP.
2. Enable Tencent Hunyuan 3D and choose OFFICIAL_API.
3. Submit a text-to-3D prompt, for example: “A simple low-poly wooden stool with four legs, centered, clean topology, no background.”

Actual result

{
  "Response": {
    "Error": {
      "Code": "AuthFailure.SignatureFailure",
      "Message": "The provided credentials could not be validated. Please check your signature is correct."
    },
    "RequestId": "2feaf401-19c6-4867-b31e-51385ab84c1b"
  }
}

Expected result

The API should return a job ID and begin the AI3D generation.

This resembles #274, but the on-disk addon already uses service = "ai3d", SubmitHunyuanTo3DProJob, and version 2025-05-13, and the issue persists after restarting Blender MCP. No secrets are included in this report.