[Bug] opencode-go provider silently returns empty response / hangs on machines with broken IPv6 connectivity

Description

opencode 1.18.11 中使用内置 opencode-go provider（OpenCode Zen Go，https://opencode.ai/zen/go/v1）时：

• TUI 发送消息后无任何输出、无报错，会话一直无响应
• opencode run --model opencode-go/deepseek-v4-flash "hi" → 只打印 > build · deepseek-v4-flash 头，exit 0，无内容输出
• 同机器上内置 deepseek provider 正常工作

Environment

• opencode 1.18.11（官方二进制，SHA 已验证）
• Debian 12 / Ubuntu，x86_64
• Network: machine has an IPv6 address, but the IPv6 route to Cloudflare (opencode.ai) is broken (IPv6 direct connection fails)

Debug findings

1. Direct curl / node fetch calls to https://opencode.ai/zen/go/v1/chat/completions → all work fine (curl has happy-eyeballs and falls back to IPv4)
2. Request capture confirms opencode sends correct params (model, key)
3. Key difference found: opencode's bundled bun fetch resolves the DNS to IPv6 and connects over IPv6, but on this machine IPv6 to Cloudflare is unreachable → connection established but stream data never arrives → empty output / hang
4. The same opencode works perfectly on another machine without IPv6 (phone Termux environment)

Verification
After disabling IPv6 system-wide (net.ipv6.conf.all.disable_ipv6=1), native opencode-go immediately works again — all models reply correctly.

Root cause
opencode's bundled runtime (bun fetch) has no happy-eyeballs / IPv4 fallback: when IPv6 is unavailable, it waits forever instead of falling back to IPv4.

Suggested fix
Implement happy-eyeballs (RFC 6555) dual-stack fallback in the bundled runtime: if the IPv6 connection fails or stalls, automatically fall back to IPv4 instead of waiting indefinitely.

Plugins

none

OpenCode version

1.18.11

Steps to reproduce

1. Configure opencode-go provider in auth.json with a valid API key
2. Ensure the machine has an IPv6 address but broken IPv6 connectivity to Cloudflare (e.g. home broadband with unreachable IPv6 route)
3. Run opencode run --model opencode-go/deepseek-v4-flash "hi" → prints only the > build header, exits 0 with no output
4. Control: opencode run --model deepseek/deepseek-v4-flash "hi" works fine

Screenshot and/or share link

No response

Operating System

debian12

Terminal

bash