five unused variables left in server code and examples #3854

Body:

Several variable and attribute definitions remain unused in the current codebase. Some were left behind when earlier refactors removed their consumers.

Unused definitions

• src/mcp/server/stdio.py: _StreamClaim.fd is populated during construction but never read. The release callback uses the enclosing function’s fd instead.
• src/mcp/server/__main__.py: logger is unused after [v2] Dispatcher/ServerRunner receive-path swap — replaces BaseSession #2710 removed receive_loop(), which contained its logging calls.
• examples/servers/simple-streamablehttp-stateless/mcp_simple_streamablehttp_stateless/server.py: logger is unused after refactor(examples): migrate all HTTP examples to streamable_http_app() #2291 removed the lifespan implementation that referenced it.
• examples/servers/simple-auth/mcp_simple_auth/server.py: app._resource_server_settings, introduced in refactor: move transport-specific parameters from FastMCP constructor to run() #1898, is assigned but never read. Despite the accompanying comment, run() receives host and port directly.
• examples/servers/simple-auth/mcp_simple_auth/legacy_as_server.py: app._server_settings has the same issue and was also introduced in refactor: move transport-specific parameters from FastMCP constructor to run() #1898.