Add proxy support for HTTPS_PROXY/https_proxy environment variables

[mkw]

Summary

• Adds proxy support for environments that route network traffic through HTTP proxies (e.g., Claude Code sandbox)
• Detects https_proxy / HTTPS_PROXY environment variables and creates a proxy-aware fetch using undici's ProxyAgent
• All existing tests pass, plus 5 new tests for proxy functionality

Problem

Node.js native fetch() (powered by undici) does not respect proxy environment variables. This causes mcpc to fail with getaddrinfo ENOTFOUND errors in environments like Claude Code's sandbox, which routes all network traffic through a local HTTP proxy.

Solution

When proxy environment variables are detected, create a custom fetch function using undici's ProxyAgent and pass it to the MCP SDK's StreamableHTTPClientTransport:

const { ProxyAgent, fetch: undiciFetch } = await import('undici');
const proxyAgent = new ProxyAgent(proxyUrl);

const proxyFetch = (input, init) => {
  return undiciFetch(input, { ...init, dispatcher: proxyAgent });
};

Changes

File	Description
package.json	Added undici dependency
src/core/transports.ts	Added createProxyAwareFetch(), made transport functions async
src/core/factory.ts	Added await for async transport creation
test/unit/core/transports.test.ts	Added 5 proxy tests, updated existing tests
test/unit/core/factory.test.ts	Updated mock to return Promise

Test plan

• All 340 existing unit tests pass
• 5 new proxy-aware fetch tests pass
• Manually verified: mcpc successfully connects to remote MCP servers through Claude Code's sandbox proxy

Fixes #4

🤖 Generated with Claude Code