fix(client): wait for output pump before exit to prevent truncated stdout

[He-Pin]

Motivation

E2ETests.secondInvocationReusesServer was flaky because the output
pumper thread (outThread) is a daemon thread. After
serverLock.await() returns, the main thread immediately reads the
exit code and calls System.exit(), killing the daemon thread before
it finishes draining the socket buffer to stdout. This results in
empty or truncated output.

Modification

Add outThread.join(5_000) after serverLock.await() in
SjsonnetClientMain.java. The server closes its end of the socket
before releasing serverLock, so by the time await() returns the
socket has EOF waiting. The join gives the output pumper thread time
to read all remaining data and terminate naturally before the client
process exits.

Result

E2E tests pass consistently across 5+ consecutive runs (previously
flaky). No deadlock or hang risk — ProxyStreamPumper reads until
EOF from the closed socket, then exits cleanly. The 5-second timeout
is generous relative to typical socket drain latency (<100ms).

References

• Prior stabilization: PR fix(test): stabilize E2E secondInvocationReusesServer trailing newline #998 (trailing newline band-aid)