MCP config read failures now distinguish an inaccessible parent directory from a real read failure

When Claude Code tries to read an MCP configuration file and the attempt fails with an EACCES error (permission denied) on a stat-type check, on non-Windows systems, it's now recognized as meaning a parent directory isn't searchable, rather than a real failure to read the file. In this case the config is reported as not present, with a distinct telemetry reason, instead of being logged as a generic fatal read failure.