Fixes #9557

When debugPrint('line1\nline2') is called, the VM sends the message as two separate stdout events: 'line1\n' and 'line2'. The existing 1ms buffer in StdoutEventHandler only handled the case where the second event is a lone '\n' (e.g. foo followed by \n). It did not handle embedded newlines, so each chunk became a separate log entry
in the Logging screen.

The fix extends the buffer logic: if the buffered message ends with '\n', the next incoming message is treated as a continuation of the same print statement and combined into a single log entry.

_StdoutEventHandler was also exposed as StdoutEventHandler with @visibleForTesting to allow direct unit testing of the handler logic.

Before: debugPrint('line1\nline2') appeared as two separate log entries.
After: It appears as a single log entry.

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