bug(meta/storage): panic on Snapshot notification in compactor observer manager

[hzxa21]

Describe the bug

RisingWave may panic when handling notifications in the observer manager path.
From production log:

• WARN risingwave_common_service::observer_manager: Stream of notification terminated.
• thread 'rw-main' (1116) panicked at src/storage/compactor/src/compactor_observer/observer_manager.rs:66:17: invalid notification info: Snapshot

This suggests Snapshot notifications can reach a code path that treats them as invalid and triggers a panic.

Expected behavior

The system should handle Snapshot notifications gracefully (or ignore them safely) without panicking, and the notification stream should remain healthy.

Actual behavior

The notification stream terminates and rw-main panics with invalid notification info: Snapshot.

Impact

• Potential service disruption due to panic in main runtime thread.
• Notification processing may stop unexpectedly.

Environment / branch context

• Repository: risingwavelabs/risingwave
• Branch where observed: pr-25706

Logs

WARN risingwave_common_service::observer_manager: Stream of notification terminated.
thread 'rw-main' (1116) panicked at src/storage/compactor/src/compactor_observer/observer_manager.rs:66:17:
invalid notification info: Snapshot

References

• https://risingwave-labs.slack.com/archives/C0846L0TZDH/p1776329763486849
• https://web.clearfeed.app/collections/6077?account=fb6b8016-a0a7-4415-8d9d-36ffcf3d3e77&request=2163

Notes

Potential area to investigate:

• Notification type handling in compactor observer manager.
• Why Snapshot is reaching a branch that treats it as invalid.
• Whether observer restart/reconnect flow can deliver Snapshot unexpectedly.