[P3][Logs] Replace per-dispatcher lifecycle logs with manager and changefeed summaries

Parent issue: #4691

Intent

Dispatcher lifecycle logs are currently emitted per object for events such as add, commit, ready, remove, drop, and handshake. When the number of dispatchers is high, the resulting logs are large in volume but low in marginal information. This task shifts lifecycle visibility to manager-level and changefeed-level summaries.

Modification points

• Replace per-dispatcher lifecycle narration with periodic or window-based summaries at the manager or changefeed layer.
• Keep counts for key lifecycle events such as added, removed, reset, handshake, and drop.
• Reserve per-dispatcher details for abnormal cases or diagnostics mode.

Entry points

• downstreamadapter/eventcollector/event_collector.go:259
• downstreamadapter/eventcollector/event_collector.go:287
• downstreamadapter/eventcollector/event_collector.go:302
• downstreamadapter/eventcollector/event_collector.go:329
• downstreamadapter/eventcollector/dispatcher_stat.go:603
• downstreamadapter/eventcollector/dispatcher_stat.go:621
• downstreamadapter/eventcollector/dispatcher_stat.go:659
• downstreamadapter/eventcollector/dispatcher_stat.go:670
• logservice/logpuller/region_event_handler.go:387

Done criteria

• Large dispatcher create and delete waves produce summary-oriented logs by default.
• Managers expose the important counts without logging each dispatcher lifecycle step.
• Detailed per-dispatcher logs remain available only for exceptions or targeted diagnostics.

Start by reading the listed entry points in downstreamadapter/eventcollector/event_collector.go and dispatcher_stat.go, then inspect logservice/logpuller/region_event_handler.go. Trace where dispatcher lifecycle events are logged and how manager or changefeed context is available. Done means large create/delete waves produce summary logs by default, with per-dispatcher details limited to exceptions or diagnostics.