fix(lsp): wait for gopls to exit before canceling its command context

Shutdown canceled the exec.CommandContext before cmd.Wait, racing a
kill against the clean exit after the exit notification. On Go 1.21+
Wait then reported signal: killed, context.Canceled, or on Windows
TerminateProcess errors, never nil. Shutdown now waits up to five
seconds for the clean exit and only cancels as a kill fallback, and the
proxy tests assert a nil shutdown error.