Mcp Router Lazy Initialization

Added :ref:lazy_initialization <envoy_v3_api_field_extensions.filters.http.mcp_router.v3.McpRouter.lazy_initialization> option to the MCP router filter. When enabled, the initialize response is returned immediately without contacting backends. Each backend is initialized on-demand when a request first routes to it, avoiding slow or misbehaving backends from blocking client initialization.