log_event_decoder: add tests for group marker skipping with corrupted data

Add comprehensive test coverage for group marker skipping when read_groups
is disabled, including:

- Normal group marker skipping (GROUP_START/END properly matched)
- Unmatched GROUP_START markers
- Unmatched GROUP_END markers
- Multiple consecutive group markers
- Mixed invalid group sequences
- Buffers containing only group markers

These tests verify that the decoder:
1. Correctly skips group markers when read_groups=false
2. Returns all normal log records in order
3. Never returns zeroed events
4. Handles corrupted/malformed group data gracefully
5. Doesn't crash on invalid group sequences

Also improve code documentation explaining that msgpack_unpack_next
handles reinitialization internally, so explicit cleanup isn't needed
in the recursive call path.

Signed-off-by: Eduardo Silva <[email protected]>

Author: edsiper

src/flb_log_event_decoder.c

@@ -407,6 +407,11 @@ int flb_log_event_decoder_next(struct flb_log_event_decoder *context,
             }
 
             if (context->read_groups != FLB_TRUE) {
+                /*
+                 * Skip group markers by recursively calling to get next record.
+                 * msgpack_unpack_next will properly destroy and reinitialize
+                 * unpacked_event, so no explicit cleanup needed here.
+                 */
                 memset(event, 0, sizeof(struct flb_log_event));
                 return flb_log_event_decoder_next(context, event);
             }