* Improve exception diagnostics for stdio client
Track the last several lines of stderr and include those in an exception created as part of CleanupAsync if the server process has already exited. This is based on an assumption that the server should never exit prior to CleanupAsync being called.
* Address feedback and fix test failure
- It allows a single MCP session spanning multiple requests
to be handled by different servers without sharing state
- It does require the servers share data protection keys, but this
is standard for ASP.NET Core cookies and antiforgery as well
- It immediately throws for unsupported operations like sampling
