Go: include CLI subprocess stderr output in error messages

[chlowell]

When the CLI subprocess fails (crash, bad args, etc.), the Go SDK returns a generic error that doesn't explain why the CLI exited. With this PR, the SDK appends the subprocess's stderr to these errors:

 process exited unexpectedly
 failed to kill CLI process: os: process already finished
+stderr: something went wrong

The implementation is a little complex because of the synchronization required to prevent races and leaks, and enable a client instance to start a new subprocess while concurrently cleaning up after an old one. The high-level gist is that the client uses a ring buffer (capped at 64 KB) to track stderr and drains that into an error message when the subprocess exits.

[Copilot]

Pull request overview

This PR improves error messages in the Go SDK by capturing the CLI subprocess's stderr output and appending it to errors when the subprocess exits unexpectedly. A goroutine drains stderr into a ring buffer (capped at 64 KB), and the buffer contents are appended to error messages when the process fails.

Changes:

• Adds a stderr drain goroutine (startStderrDrain) and a 64 KB ring buffer (stderrBuf) to the Client struct, protected by stderrBufMux and synchronized via a stderrDone channel.
• Updates killProcess, monitorProcess, and all error paths in startCLIServer and Start to wait for the drain goroutine and include stderr in error messages.
• Adds TestClient_StderrCapture with three subtests covering startup failure capture, buffer capping, and buffer clearing on stop.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
go/client.go	Core implementation: stderrBuf/stderrBufMux/stderrDone fields, startStderrDrain, getStderrOutput, stderrError, updated killProcess/monitorProcess/startCLIServer/Start/Stop/ForceStop
go/client_test.go	New TestClient_StderrCapture test: builds a Go fixture binary that writes to stderr and exits, then verifies capture, capping, and clearing behavior