dispatcher manager: heartbeat collector should validate message source maintainer

Problem

downstreamadapter/dispatchermanager/HeartBeatCollector.RecvMessages currently routes and handles incoming maintainer->DM control messages by changefeed ID, but does not validate msg.From against the current maintainer ID tracked by the corresponding dispatcher manager.

Risk

During maintainer switch windows, stale messages from an old maintainer can be accepted and executed. This is a control-plane consistency risk and affects more than recover path, including schedule/merge/heartbeat response and other downstream control messages.

Expected Behavior

For each changefeed-scoped control message, DM should only accept messages from the current maintainer of that changefeed. Stale-source messages should be dropped with warning logs.

Suggested Fix

• Carry source node in message wrappers pushed from RecvMessages to handlers.
• In handler Handle(...), compare source with dispatcherManager.GetMaintainerID().
• On mismatch: log and drop.

Scope

At least cover high-risk control messages:

• TypeScheduleDispatcherRequest
• TypeMergeDispatcherRequest
• TypeHeartBeatResponse

(Optionally extend to checkpoint/redo control messages for full consistency.)

Start in downstreamadapter/dispatchermanager/HeartBeatCollector.RecvMessages and trace the wrappers and Handle(...) implementations for TypeScheduleDispatcherRequest, TypeMergeDispatcherRequest, and TypeHeartBeatResponse. Verify that each changefeed-scoped message is checked against dispatcherManager.GetMaintainerID(), with mismatches logged as warnings and dropped.