Add HTTP MCP server support for pre-launched backends

[Copilot]

Gateway now routes to already-running HTTP MCP servers. Previously, only stdio servers launched via Docker were supported.

Implementation

Config: Added type: "http" server configuration with url and headers fields. Environment variable expansion supported in header values.

Connection: HTTP backends proxy JSON-RPC requests with configured headers. Uses reusable HTTP client with connection pooling and atomic request ID generation.

Launcher: HTTP backends skip process launch, storing only connection metadata.

Configuration Example

{
  "mcpServers": {
    "safeinputs": {
      "type": "http",
      "url": "http://host.docker.internal:3000/",
      "headers": {
        "Authorization": "${SAFEINPUTS_AUTH_TOKEN}"
      }
    }
  }
}

Requests to /mcp/safeinputs forward to the HTTP backend with the configured Authorization header. Stdio and HTTP backends can be configured simultaneously.

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