The constructor guarded its unwritable-directory case with the global
`\RuntimeException`, the only throw in `src/` still outside
`Mcp\Exception\ExceptionInterface`. A consumer wrapping server setup in a
`catch (ExceptionInterface)` missed it and got an unhandled SPL exception on
a misconfigured session directory instead.
`Mcp\Exception\RuntimeException` extends `\RuntimeException`, so this is
additive: existing `catch (\RuntimeException)` blocks keep working.
The constructor had no test coverage at all, so this adds one for the throw
plus the directory-creation and write/read paths around it.
