fix(mcp): make API base URLs configurable via env vars

[ManuelElvir]

Closes #5643

Problem

The MCP server hardcodes both API base URLs:

const HELICONE_API_BASE = "https://api.helicone.ai";
const HELICONE_AI_GATEWAY_BASE = "https://ai-gateway.helicone.ai";

EU users (eu.api.helicone.ai) get a 401 "No API key found" error because their EU API key is sent to the US endpoint. Any HELICONE_BASE_URL env var set in the MCP config is silently ignored.

Changes

helicone-mcp/src/lib/helicone-client.ts

const HELICONE_API_BASE = process.env.HELICONE_BASE_URL || "https://api.helicone.ai";
const HELICONE_AI_GATEWAY_BASE = process.env.HELICONE_AI_GATEWAY_URL || "https://ai-gateway.helicone.ai";

helicone-mcp/README.md

• Added EU configuration example
• Added environment variables table documenting HELICONE_BASE_URL and HELICONE_AI_GATEWAY_URL

How to test

1. Get an EU API key from https://eu.helicone.ai/settings/api-keys
2. Configure the MCP server with:

   {
     "env": {
       "HELICONE_API_KEY": "sk-helicone-eu-xxx",
       "HELICONE_BASE_URL": "https://eu.api.helicone.ai"
     }
   }

3. query_requests should return results instead of 401

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