[BUG] Message loss may occur during channel shutdown

[mgaffigan]

Describe the bug
SourceConnector.dispatchBatchMessage does not distinguish "channel is shutting down" vs "batch contained no messages" to callers, so messages may be silently lost during shutdown.

Caught by @gibson9583 in #453, but seems to affect all callers of dispatchBatchMessage (including TCP/File/... that has Process Batch enabled).

To Reproduce

1. Repeatedly send batches
2. On a separate thread, cycle the channel up and down

Expected behavior

• Every message should succeed or fail
• Successful messages should be observable in the channel

Actual behavior

• Messages can fail silently and are not observable in the channel