MCP tool calls timeout at 5 minutes due to undici headersTimeout despite MCP_TOOL_TIMEOUT=1200000 (20 minutes)

Environment
SDK: 0.1.65 above -> 0.1.76 as well
Host: E2B sandbox

Problem
We have configured `MCP_TOOL_TIMEOUT=1200000` (20 minutes) in the settings.json env section, but MCP tool calls still fail after exactly 300 seconds (5 minutes) with "fetch failed" error when using long-running MCP tools.

Logs:
```
2026-01-05T00:25:20.380Z [DEBUG] MCP server "deep-research": Tool 'get_results' failed after 300s: fetch failed
2026-01-05T00:25:20.380Z [ERROR] "TypeError: TypeError: fetch failed\n at node:internal/deps/undici/undici:14900:13\n at async Ho1.send (file:///app/node_modules/.pnpm/@anthropic-ai+claude-agent-sdk@0.1.65_zod@3.25.76/node_modules/@anthropic-ai/claude-agent-sdk/cli.js:1475:32554)"
```