MCP-route parser bypass for Phoenix endpoints.
Phoenix endpoints commonly run Plug.Parsers before the router. This
adapter keeps that parser unchanged for every request except the configured
MCP routes, which must reach the MCP Plug with their bodies still unread so
that authentication can run first. :mcp_path accepts one path or a
non-empty list of at most 32 unique paths. Matching uses decoded path_info
and covers each configured path's complete forwarded subtree, including
route-equivalent trailing or repeated slashes. This mirrors Phoenix
forward/4: a request below an MCP mount must reach that mount with its body
unread even when the MCP Plug will reject the remaining sub-path.