Import System package when available

[mattt]

The MCP SDK depends on the swift-system package for its FileDescriptor type. However, this creates incompatibilities when attempting to use this library with other libraries like this experimental Subprocess package (SF-0007) that conditionally import the swift-system package like so:

#if canImport(System)
    import System
#else
    @preconcurrency import SystemPackage
#endif

Trying to use both in the same project results in the following error:

Cannot convert value of type 'SystemPackage.FileDescriptor' to expected argument type 'System.FileDescriptor'

This PR replaces the existing instances of import SystemPackage with the same pattern, which should resolve the issue without any other side effects.

This PR also adds a isResourceTemporarilyUnavailable helper method to MCP.Error to eliminate the need for Client and Server to import System.Errno. Additional test coverage is provided.

See also apple/swift-system#60