[P2][Logs] Rewrite low-signal logs around control flow and state transitions

Parent issue: #4691

Intent

A large set of existing logs merely narrate control flow or state movement without explaining why the event matters. Those logs increase reading cost but do not help incident triage. This task rewrites low-signal messages so Info and Warn logs explain the operational meaning of the event.

Modification points

• Review control-flow narration logs and classify them by diagnostic intent: state transition, fault explanation, recovery action, or no value.
• Delete or demote logs that do not answer a useful triage question.
• Rewrite the remaining logs so they explain what happened, why it matters, and what the system does next.

Entry points

• server/server.go:296
• server/server.go:303
• downstreamadapter/dispatchermanager/dispatcher_manager.go:854
• downstreamadapter/dispatchermanager/dispatcher_manager.go:923
• maintainer/maintainer.go:307
• downstreamadapter/eventcollector/dispatcher_stat.go:670

Done criteria

• Default Info and Warn logs read as diagnostic explanations instead of control-flow narration.
• Logs without clear operational value are removed or demoted.
• The remaining messages make their trigger condition and action clear.

Start by inspecting the listed log sites in server/server.go, downstreamadapter/dispatchermanager/dispatcher_manager.go, maintainer/maintainer.go, and downstreamadapter/eventcollector/dispatcher_stat.go. Classify each message by diagnostic intent, then review whether its trigger, operational meaning, and next action are clear. Done means low-value narration is removed or demoted and remaining Info/Warn logs explain the event for incident triage.